Like mulveyr mentions, what's the intended use of the bicycle?. Also, what do you personally like in a bicycle? Start with the bike shop salesman's "2 inches of top tube clearance" standover fit test as a starting point and adjust from there. Personally, I like a road-style bicycle where my "junk" (can I say that here?) is resting on the top tube when I'm standing over it, and just a little more clearance on an MTB (I don't ride aggressive single track, just fire roads). And when in doubt, I go for the slightly smaller frame sicne it can be adjusted via the seat tube and stem. Your mileage may vary . . . .