Depends what you are doing, for most things an i5 750 is plenty. The Phenom II x6's only do well in a few particular tasks, but for the most part get beaten by their quad core counterparts in gaming and other tasks that arent heavily multi threaded.
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 to ask if it is worth the $90 over the i5/phenom IIx6 (both at $200 at newegg right now)
Another option is the phenom II 955 if you want to save money. At $160 it is inferior to the i5 in every way but its performance reflects its price.