Both the 35RC and the GIII QL17 have deservedly fine reputations. There are tons of Canonets out there; fewer of the Olympus, it seems. The Canonet has the faster lens, which can be very useful. The Olympus is so darn small that I have to be careful to not get a finger into the frame, which, because the lens is thin and has a traditional focusing ring, is easy to do. No such issues with the Canonet.
If you want to use filters, both cameras have real oddball sizes -- 43.5mm for the Olympus and 48mm for the Canonet. I think it's easier to find 48-->X stepup rings than 43.5-->X.
Both cameras take the 625 mercury battery, so you have those issues to deal with. Both can be used without a battery.
Regarding film, you didn't say what Kodak and Fuji you were using. But those are more or less the only game in town, if you want color. For black and white, you really should try Ilford. HP5+ is great film, similar to Tri-X.