The scale length (that is the distance from the bridge to the nut) varies a lot on mandolas and is a very important factor to how the instrument sounds and plays.
Short scale instruments are easier to play
Long scale instruments have more power and better tone in the bass range.
If you play the mandolin and want a mandola as a secondary instrument, you might want a short scale instrument. If not, you're probably better off with a long scale mandola.
I'm going to write a more in-depth artice about this soon, but this are the most essential points.
The top
The most important thing here (for all kinds of strigned instuments) is to make sure it's got a solid wood top. Instruments with a laminate top are only recommended if you're desperately short on money! This is seldom a problem when it comes to the mandola though. Nearly all of them have a top of solid spruce or solid cedar. Both these are excellent choices. You may come over other kinds of wood from time to time and if you do, ask the seller about it. There may be good reasons for an unusual choice.
Style
Most mandolas on the market today are "Celtic style" designs - relatively light flat top, flat bottom instruments with a open, percussive tone. There are some other designs though, all with their own pros and cons.