I’ve used XSI for quite a while, and I’m a big fan. I’ve just started using C4D as well, so perhaps I’m in a position to provide some personal perspectives on comparing the two.
XSI is a powerhorse that’s a pleasure to work with. It’s very logically and consistently laid out, and the bang you get for your buck with the FND package is just incredible. Consider you’re getting a top-notch renderer (mental ray), full-fledged particles, superb character animation, soft and hard body dynamics – all that would cost you quite a bit with C4D, and in some cases you won’t be getting the same level of quality.
If you do any amount of modeling, you’ll probably need to work with either subdivision surfaces or NURBS or both. For Sub-D’s, XSI is perhaps the best out there, and from my experience it’s definitely easier to work with than C4D’s HyperNURBS. I don’t work with NURBS that much. Anyway poly modeling in XSI seems to me to be a lot more advanced than with C4D.
C4D has its strong points, no doubt. It’s great for setting stuff up quickly since it has a huge number of useful presets and ready-made components (splines, shaders, rendering components, etc.). Some parts of it are easier to learn, although I must say its reputation as the easiest-of-all-3D-packages may not be true throughout all aspects of the program. To give one small example, the whole “make editable” ritual is not necessary in XSI, which gives you a lot more flexibility in revising your work, and makes a smoother workflow. Personally I find that XSI’s construction history and explorer make much more sense than C4D’s object manager.
For contemporary motion graphics work, the Mograph module in C4D has no match, not by a long shot. I can get most of this stuff done with XSI but it requires a considerable amount of scripting, which may not be up everyone’s alley. Even then, I would get it done much more quickly in C4D, and speed often breeds inspiration (at least for me). Plus, C4D’s integration with AE is still the best, although there are some helpful free plug-ins for XSI in this arena.
Don’t skip checking out XSI. Get the demo (or just shell out the few hundred $$$ it costs to get FND) and spend some time with some good tutorials, there are excellent ones at digital-tutors, 3dtutorial, 3dquakers and elsewhere. You can get help from the community over at XSIbase.com and the XSI thread on CGtalk. Of course C4D has a great community as well (Cgtalk, c4dcafe).
Hope that helps, if you have any specific questions just fire away.
– AA