I have the FN 502, Sig 322, and a chopped and threaded Mark IV. Just waiting for my can to get out of jail.
Here are my impressions after regularly shooting these over the last month:
FN 502: Great trigger (1lb 12oz), nice tall sights, completely reliable with all ammo, very accurate, 15 round mag easy loading
Sig 322: Disappointing trigger (3lb 12oz), great fiber optic sights, regularly jams every few mags, less accurate, 20 round mag easy to load but sloppy loading causes jams
Mark IV: OK Trigger (You'll want an upgrade), red dot sight (I prefer open sights), completely reliable with most ammo, extremely accurate, 10 round magazine easy to load
I thought I would love the Sig... and it's really a nice pistol for what it is, but the FN 502 is clearly superior.
It will be interesting how they perform when I get the suppressor.