It boils down to this.
Buy one that plugs into the wall.
Buy one with variable speeds (with low RPMs).
Buy a flex shaft attachment that allows you to place your heavy dremel down. I don't care how small they are, they're all too heavy and the flex shaft attachment gives you SOOOO much more control because it's so light.
And don't skimp out on bits, sanding discs and cylinders. You'll go through the sanding bits fast.
As for the make, like any tool, money talks. Buy name brand and don't be cheap.
This is all good advice, and I'll add one thing:
If you're buying Dremel, buy an older American-made model.
Dremel used to be built in the USA and their stuff was fantastic; I have a ten-year-old plug-in model and it still runs great. A few years back, they shifted their production to China and all reports I've read are that the new units have shitty torque and burn out after a couple years of use.
If you just want a good, solid rotary tool and don't care what brand, I've heard really good things about Proxxon
for hand-held rotary tools, and Foredom Electric
for larger, bench-mounted applications. The Proxxon especially are competitive with Dremel on price and I believe the bits are cross-compatible; they're also better for drilling and boring, tasks where Dremel falls a bit flat. I haven't used them myself, but I know a lot of pros do and they're supposedly great.