I have both in mine, rode across Canada with it, around 10,000 miles so far. So far so good.
Note that the difference in speed is only about 5-6% lower in first and the same taller in fifth. It drops a few hundred rpm only at any given speed, but it does make a (small) difference in the feel on the highway.
I can't
feel the difference in first, but it is several hundred more rpm at a given speed, or a little bit slower before stalling, so that's a good thing off-road, but it's not an earth shaking difference.
Assuming you have an R80G/S, or R100GS, you should be able to cruise at 70-80mph all day as standard. I routinely do it two up loaded. The engine is quite happy to rev - many people don't rev them enough!
I've even cruised mine at 90mph on the autobahn almost all day - no worries!
The R100S, with somewhat higher gearing, will happily cruise all day for a week at 100mph+ (similar to 90mph on a GS)(damhik!)
If you have the gearbox apart for a rebuild anyway, you might as well do it, but I wouldn't bother unless it needed to come apart. Remember that airhead gearboxes should get new bearings every 60-80,000 miles as routine mainenance.
------------------
Grant Johnson
Seek, and ye shall find.
------------------------
One world, Two wheels.
www.HorizonsUnlimited.com