To start with, I don't own either machine, but was interested in them at product launch. They are essentially identical and have the same engine, which on the F850 is just tuned for a bit more power. Both machines are good touring machines, with the same luggage concept as the R1200/R1250GS. Both are too heavy for real off-road use, but the genes of the F850 seem a bit more suitable for that.
The seat height and crotch length of the F850 is greater, so tall riders (> 1.80) should feel more comfortable on it.
Comparing equipment and prices, one finds that the F750 offers the much better price/performance ratio.