I recommend the XP version. It's very good, very readable, and has very simple rules. Also, it's easy to get a hold of and readily available.
The included adventure, Mr. Bubbles, is frankly among the best Paranoia scenarios I've seen. It's really
that good; my players have never had as much fun. It has just enough material for one long or two medium-length sessions.
If you like that, there are two books of classic adventures re-released for XP, too, incuding the aforementioned Yellow Clearance, Black Box Blues. I have a soft spot for ... crap, the one where you're babysitting a gigantic killer vehicle/robot. That's a fun one
As for campaign vs. one-shot... Well, just like with Call of Cthulhu, the expectation is that you'll tone things down for a long-term campaign. The XP book gives great pointers for this, with the "straight" game style. I'm personally more of a fan of the Classic, one-shot style, but YMMV.
-O