I bought a "taller" 5th gear for about $500 (used) and I think a R&P is about $2700. I installed the .642 gear. I got a good deal on it.
The guy I bought the gear from was going to the 3.77 (or maybe a lower ratio) R&P for the reasons you stated.
One (minor) consideration is that if you go to the taller 5th gear, you have to mill some material off the selector shaft. This is an incremental cost.
I was inside my transaxle already, so installing the taller 5th was a no brainer. But I don't have the 3.77 R&P experience to compare it to.
I think the taller 5th only drops the RPM by about 400-500 at 70 mph. It's not a HUGE difference IMHO.
Rocky