i've only ever driven my own 155 - a 2L 8V. my only 'complaints' - which aren't really complaints, but inherent traits with this model - are the crashy ride quality and fairly 'rough' engine (likes to rev and all that, but isn't very smooth).
if i had the choice, i'd buy a Q4 at the drop-of-a-hat. though i'd want to research it carefully, cos a lot of comments suggest they are not exactly reliable!
secondly, i'd love a V6 version, for both the extra power and the lovely V6 sound.
thirdly, i'd like to at least test drive a 16V, to see if the engine is much more refined or not (with its balancer shaft). if it's anything like the 2L 156 i had, it's a lovely smooth engine.
i'm also a sucker for climate control, which my car has. so any model with that, could sway me. other niceties would be sunroof and leather trim.
but as suggested, if you have a choice, test drive what you can to help decide. however, i really do love my 8V. it's fun to drive, has decent creature comforts, and so far, easy to 'work on' in terms of mechanical upgrades and replacing parts.
i hope you find something. and i'll be envious if it's a Q4.