MASSIVE GENERALISATION ALERT!
Floorstanders typically offer more bass than standmounters (more volume in the cabinet; space for larger drivers), while standmounts typically (as they usually have less drivers) beat similarly priced floorstanders for imaging and integration.
BUT there is far more difference between a good and poor speaker than between floorstanders and standmounts, so don't get too hung up about it. Find a speaker you like the sound AND look of, and that suits both your system and where you intend to position it in the room.