Congrats on taking the plunge. You will LOVE having a winch. I looked up your tractor specs. I have a similar size machine, a Kubota MX4700 and a Farmi 351 winch, which has the same pull capacity as the smaller tajifun you're considering.
I bought my winch used, and I have at times wished for a larger one. That said, even with only 7700# of pull, that winch will yank that tractor around pretty good at times. A bigger winch would be nice occasionally, BUT it'd put more stress on the machine, and have the potential to really pull the tractor around. Furthermore, the bigger heavier winch can pull more (well, if you can anchor the tractor well enough) but it reduces the amount you can pick up on the three point. Because it's heavier, you'll be a little more limited in how large of a log you can pick up.
You still have to decide for yourself, but I don't think you'd be disappointed with the smaller one. I've pulled out some pretty big stuff with my little winch. Of course if you can see yourself buying a bigger tractor down the road, going for the larger winch now and being careful not to over stress your tractor would be a good option. You could even adjust the clutch to limit the pull to a number similar to the smaller winch.
Out of curiosity, is there a particular reason for going with the tajifun? I know they make good stuff, but there's other good winches too for a fair amount less money.
Anyway, here's a big one I yanked out. It's a red oak log that was about 34" on the small end. Winched it no problem on ground with 2" of mud on top of the frost. Lifting it and driving away was a little more challenging.
