Do you have a preference for da/sa, striker or hammer?
How about an alloy / metal frame vs polymer?
Here are the nines that I have had any experience with:
Brother's Glock G19 - liked the compact size and weight, didn't really enjoy the initial impression of shooting it, but I think with time I could warm up to it
Friend's FN FNP9 - instant winner, I cannot say enough good things about this pistol and recommend it highly
My SIG Sauer P226 (old German one, holster wear, no rail) - bought it used. Liked the mechanicals from day one but didn't warm up to shooting it until I found the right grips. Now, I doubt I'd trade it for anything at any price
Friend's Ruger P(89?) slightly older da / sa anyway... enjoyed firing this one. Nice feel and mild recoil
All above were purchased under the $500 mark, I believe the P226 is the only one to pass the "limp wrist test" which simulates defensive fire while injured.
Ex - Navy, Persian Gulf Veteran. Loved shooting the M14, 1911, M60, M2