640, it has C3 stepping, rather than C2 which the 635 has, meaning better power efficiency, and much, much better overclockability.
You can tell which AMD processors have C3 and which have C2 stepping by the product code. Those ending in IBOX have C2, those ending in MBOX have the much better C3 stepping
99c extra for .1GHz extra would be pointless, but if it was $10 more for C2 over C3 (if you plan to OC) it would be worth it