For my CCWs, the XS "express" with tritium. IMO, nothing is faster to align at "social" ranges. Works equally well in bright or low light and is at least as rugged as other fixed systems.
The "shallow vee - large bead" concept, while developed for and proven on large-bore rifles for dangerous game at short-to-moderate ranges, is every bit as practical and sound when applied to a defensive sidearm. No real surprise; the situations are very similar.
Bonus points were awarded for being able to get better hits sans corrective eyewear than with any other conventional open iron configuration. No small consideration, at least to this G.I.T. (Geezer-In-Training).