You'll find a vast variety of personalities in either section. The "rivalry" really stems, near as I can tell, from Spam pretty much encapsulating the entire "giving a fuck is bad brah" attitude and thus views the sheer effort and occasional shitstorms rising from differences in opinion in Off-Topic as being way too over-the-top-serious. Meanwhile, those in Off-Topic (who, generally, tend to think of themselves as intellectuals) find that attitude immature and beneath both them and their serious conversation. 

That's talking in gross generalisations, though. If you feel like a genuine debate or discussion about a serious topic, with the kinds of considered responses that are rare in most communities on the net, go to Off-Topic. For general lulzies, Spam is a better bet (though seemingly not by much, these days.) Just don't cross the two, and you should be able to find something worthwhile in both places.