... To answer your question,
I'm an XSI user. From my experience, XSI will more than satisfy your prop modeling desires for any Source game (if you end up wanting to animate something one of these days). Max is a more popular choice, and even though I, myself, have next to no experience using that modeling app, there are still a couple of folks here that are pretty skilled with it that you could go to for help.
Those two are probably the two you would want to pick from because, a) LOTS of folks use them, and b) both of them have extensive Source support (.SMD exportation, character rig guides, the whole shebang). The only other program from that short-list that I'm at all familiar with is Blender, and while Blender is wildly powerful (and free!), its interface is not very intuitive. Blender would surely suit your needs (I know some folks at Interlopers that are die-hard Blender fans and have the work to prove it), but it would take you a while to learn how to use the program before you became facile enough to produce some real results, especially as a beginning modeler (at least, I assume you are so). Also, if you wanted to do anything more than prop modeling, I'm really not sure how well it would support the filetypes that Source needs to successfully compile animations.
If you don't really like using XSI or Max, do some research about how well other modeling apps support Source, and find one that suits you comfortably. Interface is a pretty important thing, so XSI and Max have my vote, followed closely by Blender, but everyone has their own taste, so you'll have to do some shopping around!