If you cant figure that one out on your own you shouldn't even be buying a processor.
Of course the Phenom II X4 940. It a Quad-core chip, excellent overclocker and has a high stock frequency
The PII X2 550 also has a high freq. and is a very good oc'er, but it's a dual-core chip and it will only make sense if you're on a tight budget and not doing much multitask
They both have 6MB L3 cache