There are 3 basic types of prep that are easily obtainable: Top/Roving/Sliver (although I don't think any vendors sell real sliver), batts, and rolags. There's also mill-ends, but those are basically top, so that's where I put it. Each type of prep is ready to spin, but some are easier than others (at least, for me.)
I decided I *won't* name names here - it's not worth it. Most vendors are great; some are just so-so, but those are few and far between. A vendor I love may be one you hate...so I just won't go there. Assume the vendor you're looking at is great until you've had a bad experience with them - and I include eBay vendors in this statement.
Top/Roving/Sliver is probably what most newbies start out with. It's easily obtainable, relatively easy to spin, and comes in all sorts of fibers and colors. The fibers are usually parallel, giving you a worsted-type yarn. You can choose your roving by the breed, or by color, or whatever - and did I mention that you can get roving *everywhere*?
New spinners are told to split the roving (I'll use this term for ease of typing) length-ways, pre-draft it, then start spinning. You can get a decent yarn that way. You can also split it into shorter sections (each about length of the fiber), roll it into a tube, and spin that (from the middle of the mass) for a woolen-type yarn. You can also cut to the chase, and start spinning directly from the box it came in. Not that I do that, of course. *ahem*
There are various reasons for each technique (I won't bore you with that right now. Unless someone asks. *g*) I try to choose the one best suited for the yarn I want to make. If it's a hand-dyed top (like the *beautiful* one I received Saturday), I think about *what* the yarn will become, then decide from there...this batch? I think will be socks; it's Romney wool, which is long-wearing, it's next-to-the-skin soft (OK, next-to-the-feet soft, which is slightly different) and the colors are fabulous.
I have 3 choices for it: a)split it lengthwise a few times, and try to spin 2 bobbins as close to matching as possible, so that a 2 ply will have lots of solid color blocks; b)split it as above, but random spin so the 2 ply will be "accidental" (which looks great knitted) or barberpole; or c)split it, again as above, spin it fine and tight, and navajo ply it so the colors stay very defined. I'm thinking I'll go with c) on this one, but we'll see.
Mill-ends are bascially the same, but are leftovers from commercial set ups. I find them a bit more compacted, and, because the commercial mills don't buy by the fiber but from the wool pool (which may have Suffolk, Romney, Dorset, and crossbreed wool all mixed together), the fiber lengths are all over the map and it's harder for *me* to get a nice consistent yarn. It's certainly *possible*, but I have to concentrate a bit more. The cheap price makes them *very* appealing ($6/pound is *great*, but there are drawbacks to the low price. Just be aware of what you're buying, is all I'm saying.) Beginners might find the mill ends to be a bit harder to work with.
Batts - usually come from a small producer, it's the prep that comes off of a drum carder. They're usually fatter than they are long; the fibers are pretty much parallel, and, depending on *who* prepped it, they can be very dense. I *love* spinning batts, if they're well prepared - you can spin straight off the carder and get a nice yarn, you can roll it into a large sausage and spin woolen, you can strip it....they are fun.
However, some vendors don't really know how to properly utilize a drum carder. I got burned a while back....the batts were advertised as a blend of 3 or 4 fibers (all of which I *love*), the colors seduced me...but the batts were disappointing. They were what I would call "rough carded" - they had been run thru the carder maybe 1 time, the fibers were sort of stuck together, but not blended, and ...well, it was a disappointment. I *didn't* express this to the vendor - it was a relative newbie, and I was trying to be nice, but I spent a good 3 days trying to decide if I wanted to take the time to card them properly or not. (I decided not to - my carder is currently covered in dog hair, and...well, it's going to take a lot of work to get all the canine fluff off of the teeth. Since I'm not done with the bags, yet....and, I paid a pretty penny for them - I didn't *want* to have to do any work on them to make them more spinnable!) I consider "properly carded" to mean at least 3 trips thru the carder, splitting the batt into smaller sections each time and shuffling the order I feed them in. That makes for a more cohesive batt. The yarn I spun from the above mentioned disappointing batt is usuable, but it's definately a "novelty" type, and it's not something I enjoyed spinning. I had decided to sell that yarn before I even got the batts, though, so it's OK....lesson learned! (And, to see this particular finished yarn, go here. Heck, go ahead and look at my yarns page while you're at it, and critique me. I need the feedback, so I can make changes in my spinning. I got too much yarn around here and need to move some of it out! *g*)
I like my fibers well-blended. I *don't* like spinning fiber that will give me clumps of this, then a big batch of that, then a blend, then back to this....it's frustrating, especially when you're talking silk/mohair/wool. The fiber lengths are all over the place...it's not fun. The next time this happens, I'll contact the vendor and either send it back or just go ahead and recard it myself.
Now - rolags. Ah, the joys of spinning from a batch of handcarded rolags simply cannot be expressed. You get a true woolen yarn from them - lofty, bouncy, squishy....and long-draw is *the* way to speed spin. However, I've never seen any vendors selling handcarded rolags (hmmm....might be a good marketing idea!) (Punis don't count - that's cotton, and I'm only talking about wool here!), which is a shame.
Handcarding is not a hard skill to learn, but it takes lots of practise to get it right. When it's right, though....wow. It makes for easy, fast spinning - and your consistancy will go thru the roof (once you master long draw, which isn't hard, but takes practise. The secret is to pull the fiber back in a long, sweeping motion, staying .just.ahead. of the twist. Hard to explain, easy to show.)
The drawback? The time spent carding the rolags. It takes me a few hours to get a basket full of rolags that will take me a 1/2 hour to zip thru. It does make for a good rest break, though - I have to stop and card some more fiber up...it's also good at demos, because you have to explain what you're doing.
Now, you can get bad prep from any vendor - I've had some rovings that were so compacted (one was even *felted*!) from the dyeing process or the shipping process - but, generally speaking, rovings are the most consistant way to buy fiber. They're usually well prepared and spinnable as-is. Batts - be sure you either know and trust the vendor, or spend a lot of time *looking* at the batts you're thinking about buying. Most vendors are pretty nice - they don't want to sell bad prep, because word *will* get out and their business will suffer for it.
Anyway...to sum up: Roving is good, especially for beginners, batts can be *wonderful* if they're prepared properly, buy my hands-down favorite is spinning handcarded rolags. And it wasn't even *that* rant-y!