The 3640 can forward 50-70,000 pps, and the 3660 can forward 100-120,000pps. However what if that 3660 died, you have no redundancy. Having 2 routers, running OSPF and HSRP should provide redundancy and load balancing that one router can't. Of course this depends on your design, but as a general rule 2 is better than 1 (scalable, redundant, load balance etc). Money may be the deciding factor, but if your performance needs can be meet by a lower end router I would get 2 of them instead of one 3660 (if mission critical).
All of this should be taken with a grain of salt as I haven't seen your design, but as a general rule HW redundancy is a key goal.
Hope it helps.
Steve