There are quite a few F31R's on the Great Lakes, I would guess at least 10, but not all racers. Some F31 ultimate cruisers, ect. As to comparing the 31 to the 28 the 31 will handle a family better, especially a young family with room to grow. If you want to race I wouldn't go for the F28 aft cabin boat.
There is a large difference between the R and OD config on the 31. Larger Main, larger jib, canting rig, different boards, faired ama's. The F/C31R rates -30, while the C31R1D rates -60's.
would complement your F18HT well, although I'm not sure which one you will sail more.
F31R Triceratops (weekend boat)
F18HT USA-16 (weekday boat)