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Just to expand on Edison's mention of the performance difference, a 1921 is rated at 290 Kpps while a 3560x is rated at 65.5 Mpps (24 port) or 101.2 Mpps (48 port).  Full duplex gig requires about 3 Mpps.  (Cisco recommends the 1921 for up to about 15 Mbps.)

I would always use routers or Metro switches on WAN or MAN links as they provide far more control over traffic at these aggregation points.  LAN switches (L2 & L3) are excellent in the campus environment but lack features when connected to the WAN.  I have worked with a lot of customers that do exactly what you are proposing - connect the Ethernet WAN directly to the switch but I would recommend against it if you have the budget.

Andy

Thanks, that's all very helpful. The one link is 1gb and looking at the prices I'd need to spend £10k to get a router to do this. Juniper seem to do one for £1,400 though so I'll look into that.

Thanks.