Most are generally stiffer than rounds, however there are quite a few brands that are compareable to rounds in stiffness. Thomistik-Infelds are the loosest flats on the market. You will probably have to add relief into the neck if you switch to these, unless you have played with extra light guage rounds like 40-95 or 45-95 guage. D'Addario's are medium tension and if you go down a guage size you may not have to mess with the truss rod.
Example; If you generally use 45-105 in rounds, get 45-100's in D'Addario's.
GHS flats are a bit stiffer and RotoSound and Fenders are the stiffest!
LaBella's in a lighter guage aren't too bad and the sound they give is well worth it, but at $36.00 a pop make sure they are what you want. D'Addario Chromes and Half-Rounds are the most reasonably priced for the sound. They are not like old-School flats, but they are nothing like rounds either. I hope this helps.
P.S. Half-rounds are sanded down roundwounds which are then polished. They sound like flats with more sustain or rounds with less highs and treble. They do have a nice unique warm tone and they do offer a good alternative to flats and rounds.
Flats! 
