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The 3750 may not have more useful performance, in fact, if you stack them, it may have (much) less (StackWisePlus's 64Gbps vs. StackWise-480).
On some switches, it appears Cisco uses the same fabric in different models, but you only need the bandwidth to support your ports. Both the 3750X and 3850, with 24 gig ports, support dual 10g uplinks. So, total fabric bandwidth needed is (24 + 20) * 2 = 88 Gbps. If the fabric provides that much bandwidth, you're got as much you need (or that you can ever use).
Where the "excess" bandwidth might be needed on these switches is to support the stacking ports. On the 3750X, that would require an additional 64 Gbps. On the 3850, that would require an additional 480 Gbps. (From Cisco specifications, it's unclear whether either switch's stacking ports "counts against fabric bandwidth, as do the other ports.)
