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.