Take nightcrawler out of your options. Not worth at all. If you have some RUT you can give to x23, then I would use the MRUT on Gambit. It is really up to you, You probably won't be able to T2 both of them with on x-gene sub. while x23 is good, she is not really meta. Gambit offers you really high DPS but low defense, so basically a glass cannon.
I would go for Gambit, but that's just me. Do what your heart tells you :)