That's like asking should I get a ford pickup? or should I get a mustang?
How do we know? You never told us what you need it for.
Generally, you get what you pay for. You are looking in all different priceranges and whatnot. The x6 will be more powerful in anything mutli-threaded and is worth it if that is what you need. The i5 is the most powerful quad core under $200 (just barely under $200 for the 750). Per core it is still a bit more powerful than a phenom II core at 3.5GHz for most things.
At about $290, choosing the i7 870 vs the phenom IIx6 is a bit of a tough one if your uses can use the extra 2 cores. The i7 870 will perform better even in some tasks that can use the extra 2 cores of the x6 so is the better bet but you need...