You should consider what you do most with the rear end of the car. If you change wheels more often then the F104 Pro diff should be your best choice. If you change spur gears or adjust your diff setting more often, then stick with the F103 diff. Both diffs can be adjusted to whatever setting you want, and the alloy versions of both weigh about the same.
In my case I prefer the F103 type as I use the diff setting as a tuning adjustment and change gear ratios depending on the track. When I ran the F104 Pro diff I found it annoying that I couldn't tweak diff settings like the F103 diff, with the F103 diff I can run a lap or two before race start then adjust the diff in seconds trackside, something just not possible with the F104 diff.
I usually only use one set of tyres at a meeting, so the ability to change tyres without having to reset the diff isn't important to me.