3550 24 DC SMI uses an external DC power supply and does not support advanded routing protocols (static and RIP only) 24 10/100 Base-T and 2 GigE (GBIC) ports.
3550 24 EMI uses internal AC power supply and supports advanced routing rpotocols such as OSPF and EIGRP. 24 10/100 Base-T and 2 GigE (GBIC) ports.
3550 24 FX SMI uses internal AC power supply and has 100MB Fiber ports and does not support advanced routing protocols. 24 100MB FX and 2 GigE (GBIC) ports.
3550 24 PWR uses internal AC power supply and can supply 48v power for IP phones and access points if needed (negotiates power needs on a port by port basis). 24 10/100 Base-T and 2 GigE (GBIC) ports.
3550 24 SMI uses an internal AC power supply and does not support advanced routing protocols. 24 10/100 Base-T and 2 GigE (GBIC) ports.
both switches can route, but without the EMI version you must use static routing or RIP.