Out of curiousity - I just sent an email with a 1.1MB file with WiFi enabled and without WiFi enabled in a 3G area (I'm on O2 in the UK). With WiFi enabled it took 50 seconds to send, without WiFi (ie with 3G) it took 5min 10 sec.
So - at least on O2 3G, I can get much better throughput when I have a WiFi signal.
It really depends on the internet connection behind the wifi. 144k DSL is going to get smacked around by 3G, while a 12G cable connection is obviously going to be much faster than 3G.