Both are excellent rounds. The 40 usually results in more rounds per magazine, but I’d not feel under powered either way. I shoot and carry both. Personally, I prefer the 40 but the 1911 shooting the 45 is just a smooth firearm.
A 9mm is as good or better than the .40 for self Defense.
I use Win Ranger Talon 127gr +P+ and its 17 shots vs 12 in the .40 and with The ranger Cartridges its Got the Power of the .357 Sig base loads with 5 more rounds.
Penetration is 15” and 98% Bullet weight Retention with Expansion around .70.
Recoil is less and Controllability is better than .40 , .45 or .357 Sig.
The 9mm is FINE with the right Loads.
1911’s are Like Chevy’s - Their Great as Long as You always Like Tinkering under the Hood.