220 has a better CPU, power usage will be higher, and more networks ports are the main differences. Either way, depending on revision, you'll want to boost to more RAM. 2GB would be the bare minimum.
Watt is typically meaning Watt-Hours, so this device will consume 70 Watts/hour (W/h) at full load, 27 W/h on standby.
Assuming latest HW rev's, given the choice between a 120 and 220 for home use (I've used both in production), I'd go with the 220 myself. 220 won't be as heavily loaded for home use so will consume closer to the standby rating, and if you have a faster connection (50Mbps or better) and want to turn everything on, you'll want the 220.
I am planning to buy one for home use and can't decide on which one to get, mainly thinking about power use.
I plan to have a Cisco WS-C3750-24PS-S, a WAP, and have a Windows home server.
If anyone has something similar and can advice on how much power is typically needed to run this (or how it affects your bill), I will appreciate the info.