used is often a crapshoot as to whether it's any good or not.
Rebuilt really depends on who is doing it. There are some decent places, and others that are selling "rebuilt" motors using crap imported parts at massively inflated pricing.
Not my personal experience, but from seeing lots of different threads and facebook posts and whatnot.
These are a relatively basic motor, so doing a full rebuild isn't that complicated.
It also depends a little what block you have installed. If it's a GEP block and nothing wrong with the block rebuild it. If it's a GM 6.2 definitely replace. If it's a GM 6.5 you might go either way.
Also if either of your NA trucks is a 3 speed now would be a good time to upgrade to a 4 speed, buying a used engine/transmission combo is usually a good upgrade.