IMO any generalizations about the two types, with respect to bass or imaging, are just that -- generalizations. Especially the imaging part. The choice is mostly a matter of (a) budget (since bookshelf speakers generally are less expensive than floor-standers in the same product line) and (b) convenience in placement.
It's easy to add bass extension with a subwoofer(s).
Poor imaging is harder to fix (if it's due to limitations in your speakers).
Although, there are floor-standers with excellent imaging.
A bookshelf or table tends to be a bad place for speaker imaging, so for high fidelity you probably want them on stands positioned out a little into the room. This is tantamount to a pair of 2-piece floor-standers. My floor-standers do weigh 70 lbs. each, so for those times when I need to move them, I'd have been better off with the stand-mounted version.