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"The catalyst switches support only Ethernet."
Actually, the 6500 series can support serial WAN connections using a FlexWAN card.
The 6500 series can also provide other special functions, via services module line cards, such as firewall, VPN, network analysis, etc. (see http://www.cisco.com/c/en/us/products/interfaces-modules/services-modules/index.html), unavailable on the Nexus.
The Nexus offers more bandwidth capacity (although the 6500s have somewhat narrowed the gap with the sup2T) and the Nexus had its FEXs (but 6500s have narrowed this too with the new IAs.)
As Reza notes, the Nexus is oriented for large data centers while the 6500 (and subsequent 6800) series are for more "generic" L3 switching, for LAN core, distro, and host edge. (NB: although the 6500 series can do some MAN and WAN, its "router" sibling, the 7600 series, is more oriented for that.)
