I've *owned* both.
Simply put, the F6 is THE BEST 35mm SLR I've *ever* used. Period. Combine it with the latest G-series(ya, the "neutered"(no aperture ring) ones) lenses, and you are in 35mm HOG HEAVEN!!!
The F5 is wonderful too. If you want FAST slr shooting, and feel like having a beatin-down-the-zombie-apocalypse, then-take-pictures-to-document-the-aftermath camera, the F5 would be my 2nd choice(well after my 2 F4 bodies )
But in hindsight, the F6 was a very straightforward tool. The integrated data back allows for many options(of which I never used any), just read up on it.
ONE drawback(to me): the F6 takes CR-123 batteries. If you travel, you'll want to bring a couple sets of spares. The F/4/F4s & F5 takes AA batteries if you have the AA battery trays for each. AA batteries are plentiful and can be found worldwide, in the smallest of shops. CR123's, not as easily. But to me, that's a VERY MINOR issues. But one to not be forgotten IMO.