I think the main thing with fluoro is that you have to fish it with a moving bait. Either slow retrieve (it's stretch will help with the 'suck in' process that was mentioned rceently with solid tips), or with anything up to a fast retrieve - where again it's stretch is the bonus. Fishing 3, 4, 5lb braid there's a chance that a decent fish ramming in to your bait will provide enough of a shock to break you. Fluoro should provide a bit of cushioning. Braid's weak point is it's shock resistance - which has been mentioned a few times before. Its probably unlikely at this point that many of us are regularly hitting fish powerful enough to cause many problems, but if you start finding big pollack, bass or even bream (in my dreams!) then fish (and lure) loss may become a reality - this is on the faster retrieves by the way - won't be too many problems unless you and the fish are pulling quickly in two different directions - which is unlikely to happen at slow speed.
If you're bottom bouncing a lightly weighted soft bait then it's likely that fluoro will be absolutely pants. Time for the braid as soon as it becomes essential to feel what you're doing (feeling for bottom, shakeyhead, on the drop even).
If you have a choice of just one then I'd go for the braid every time.