I went the 16/18 route.

My reasoning was I did not want to raise the rear of the bike any higher, and with my wife's bad back I wanted the better ride.

 

Besides increased ride height, a larger diameter wheel/tire will give a better ride over small road variations, as there is a larger rolling circumference.

 

Did you have any fitment issues between the tire and the belt?

 

That is obviously related to section width and not rim diameter.
This is my actual experience
a 140 -70-18 Metzler is plug & play
a 150 -70 -18 Metzler required a .125 shim to move the rear pulley outward.
Remember not all tire brands are the same actual dimensions despite sidewall markings. YMMV

 

16" x 21" on mine.

 

same here...

 

Nobody had any fender clearance problems, bottoming on big bumps?

 

Height difference from 16” 90 series to 18” 70 series is minor , no issues here, even 2 up
Remember it’s only the radius dimension NOT the diameter

 

in comparing stock sized tires, there is slightly more than a 1/2''' difference overall in the height of the tire (which is only going to raise the rear end abou 1/4'')

 

Exactly
thank you
that 1/4 inch height difference will NOT create any fender clearance issues