I had 2.56 gears and switched to 3.55s last fall, but found the rpms a little high on the highway. Speedo not corrected yet but I estimate about 3300 rpm at 60 mph (and who drives 60?)

My wife and I have started an annual "topless cruise" each June with my brother and his wife. So although most of my driving is in the city, we still enjoy taking it on some long runs.
So going to the 15s will help drop the rpms on the highway.

Also, I want to add disc brakes down the road and I'm told they're readily available with the 15".

I understand the ride might be a bit better as well, but that wouldn't be a reason (for me) to change to 15s.