The two basic types you're concerned with are orbital (a.k.a. 1/4 sheet
sander, palm sander), which has a rectangular pad, and random-orbit,
which has a round pad that orbits and spins.

An orbital 1/4 sheet sander has the advantage that it will fit into
corners. It leaves an OK finish with finer grits (150+), but if you try
to get aggressive, you'll be punished with the dreaded squiggly
scratches.

A random-orbit sander is more effective all around: better finish with
fine grits, faster stock removal with coarse paper, better dust
collection when connected to a vac, less vibration for the user.

If you absolutely can't get both, I'd say get the random-orbit and use
a hand sanding bolck to get into corners.