I have a 5 screw Pre-29 4”, a 29-2 6 1/2” and a 29-2 8 3/8”.
Shooting magnum loads in the 4”, you don’t have to sniff the end of the barrel to see if it went off! The 8 3/8” handles magnum loads well but the 6 1/2” just balances better shooting the heavy hitters.
So, for what it’s worth I’d go for the 6 1/2”.
As always, your preferred activity with it will influence barrel length most or if you're just a guy like me who goes mostly on aesthetics the 4" M29, to me, is really hard to top. Personally I dont think the recoil is awful and I'll more often than not shoot 100-150 rounds a trip with it. And besides, it's just the sexiest gun.