So far I've tried:
MasterCarve by Staedtler,
Speedy Carve (also called Speedy Stamp?) by Speedball, and
SpeedyCut by Speedball. Here are the results of my *very* beginning carving skills on the three different blocks.

A general review of the process: This is really hard! DANG! One tip: when making curves, turn the stamp and keep the cutting tool still. I'll post a tutorial soon.
Keeping in mind that I'm a beginner and carving for stamping, not for printing purposes AND that I'm not mounting these guys AND that the lino blocks seem scary to me, here is my review.
Review of the 3 blocks:
Mastercarve:Cost: about $5.50 online for a 4x6 block.
Pros: very soft and easy to cut. Also, really thick. Some folks say you can carve on both sides. The white is pleasant.
Cons: too soft, really...not much crumbling but so easy to cut too much.
Speedy Carve by SpeedballCost: about $5.00 for a 4x6 block.
Pros: harder to cut, but not too hard. Carvings came out better, maybe because I had more practice but I also felt like I had more control.
Cons: I don't like the pink and I don't like how thin it is. I felt like I might cut all the way through by mistake.
Speedy Cut by SpeedballCons: So crumbly!
Pros: cheap! About 2.50 for a 4x6 block.
So that's what I've tried so far.