I'd leave it between a Sun and an SGI for practical values. I think IRIX is more fun, but Sun iron might be more practical if you're trying to pick up more widespread marketable skills.
You can get an SGI 02 on ebay pretty cheap, they practically give away Indys now. An 02 is a good starter box though. I used to have an indy, Indigo2, and an Onyx. They've all sense been sold though. In fact, in my computer setup picture on the AI community site, you can still see all three SGI boxen
