If I had to choose, I'd pick the 165 FIS. But it's close. Both great. Maybe just as good.
Just for recreational skiing, I have and strongly recommend both, 165 FIS SL (Mine are Rossi, from
@ScotsSkier, and at his recommendation - tops); and 157 FIS SL (Mine are Atomic doubledeck, very thin edge by now; a former woman racer's ski - also incredible).
You can't go wrong with either of these lengths, surprisingly - as near as I can tell,
provided they are tuned properly, especially with at least a level edge fore and aft, rather than (base) edge high fore and aft - a no no. (Tip edge high and/or with a reverse-tapered base bevel at the tip/tail can make them slightly hooky and a touch unstable - mine were briefly before I fixed them.*)
When I first got the 165 FIS, they were perfect. Man, big smiles.
But I have friends who are race coaches and such, who swear by the woman's FIS 156 to 157, also; and are really big guys. And for recreational use or coaching use, they swear by those woman's spec skis as well as the 165 FIS. I experience why every season now.
The earlier post that said the side radius isn't all that different is my experience also. And often, one can find the women's spec ski at a ski swap priced very inexpensively.
*(For example, a racer might have his SL base bevel set at a shallower zero
° underfoot, then gradually increasing fore and aft to a steeper .5
° tip and tail. But a SL ski can easily get worn or poorly tuned to the reverse, a steeper base bevel underfoot.)