My 8 shoots everything so good I don't even pick up my other 223's. I keep a bunch of 55's on hand just in case and to use if I ever decided to drag a slower twist rate out but I've not used them on anything since I've started using 75's.
After shooting 75's I've not found anything that lighter bullets do better so I just keep buying and using 75's. Paper, steel, animals, etc. I've not seen a downside.
My friend with his 7 twist only shoots 55gr bullets. He only shoots factory ammo and he has stuck with 55's. I've seen him shoot quite a few 1/2" 3 shot groups and sub 1" 10 shot groups from a 16" ar.
If I have an option of fast twist or slow twist hand me the fast one because it gives you more options.
That said, if a man wants 2 rifles by all means buy 2 rifles.