There's many criteria one can use choosing a core device, but since such a device, by being at the center of your network, may carry the most traffic, performance is often given additional weight for core device choice.
With regard to making a choice on some X number of users, choice of core is often made more toward bandwidth usage of core ports. There's often a large difference between the nomimal bandwidth of a port and the substainable bandwidth to/from a port. (E.g. the difference between a 6500 with Sup32 and 6148 10/100/1000 Ethernet vs. 6500 with Sup720 with 6748 and DFC 10/100/1000 Ethernet. The former is suited as an edge device, the latter more suited for core device.)
A stack of 3750 might be used as a core for a very small and/or light usage network. Consider that a single 48 port 3750, I believe, is not an every port wire rate device, and the performance limitation of the stack ring. However similar performance limitations are also true for certain 4500 or 6500 hardware configurations.
Although performance is often a major factor, other considerations, such as other features, might be important too. For instance, a dual 48 port 3750G stack might be a viable choice vs. a 6704 with dual Sup32s and two 6148 line cards, but the 6500 likely will offer features not available with the 3750. For instance, believe 3750s only support 32 HSRP groups and don't support GLBP.
