I use both F1 & F2. Here's a quick snap of the differences:
In general the F2 is more refined with better quality knobs, levers and parts although the rest is the same (the monorail is black instead of chrome).
The F2 has more robust and accurate engravings for rear tilt functions and so is easier to use. The rise knobs are larger and easier to control - the rear one is huge and not as insignificant as the tiny ones on the F1.
The rear international back also has a more refined grafloc style back. It is spring loaded, however unlike the F1's can be eased out with tension, and then held open with light finger pressure to slot in wider than 5x4" DDSs, or easily pushed to fit on a different back.
Both swings on the F2 are easier to use with less force - the movements are frictionless when unlocked and fully locked when locked.
Maybe my F1 is just loose, but it's not as robust in operation as the F2. Some might say those upgrades are slight but after using them for ages, I like the F2 for precision work and the F1 for slinging around when jogging
If the F1 is there for a good price, go with it - it's good enough to start off 5x4" with.