You would need to ask Cisco what they recommend, but likely they would suggest another pair of 6500s.
However, from what you describe a dual stack of 3750Gs might be just fine as well as a pair (although if you use a pair, not stacked, you might consider the 3650G).
As Glen notes, expected resource demand is a consideration since the switch's fabric is "only" 32 Gbps, but there's also its PPS to consider too. Further, in a stack configuration you need to consider that limitation of the ring bandwidth (NB: the 32 Mbps ring is really dual 8 Gbps duplex.) All of these considerations, though, I suspect would not be an issue for what you've described.
Within the 3570 series, the 12 SFP port 3750G-12S offers SDM templates not available in the other models which can be useful in a distribution or core layer role (BTW, if you mix this unit in a stack with other models, you can't use the additional templates.)
One advantage of a stack, you can Etherchannel to different stack members which might avoid some uplink usage issues working with a pair of devices.
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PS:
If there was a need for additional performance, there are other possible platforms besides the 4500 and 6500 chassis. The 3560-E, 3750-E and 4900 series could be considered too.
