45 super would be the way to go, but a 25-30# recoil spring is WAY over doing it, youd spend more time trying to cock the weapon than shooting it. i run a 18 1/2lb spring in mine with a full length guide rod, and after over 2000+ rounds of hot 45 super flying thru it, i dont have one single solitary sign of any kind of frame and/or slide battering, and have had absolutely no problems what soever. when i originally put mine together, i used the called for 22# (i think it was) spring, and it was way over sprung. hard to cock the slide,the occasional stovepipe,etc. once i switched to the 18 1/2lb spring, no problems at all even with the hottest 45 super.
you decide what spring you would use yourself of course, but personally 18 1/2 is all mine needs