If you have Wi-Fi turned on, all data traffic goes through that pathway except for phone calls and text messages. This includes file transfers. If you turn Wi-Fi off, then data is passed over 3G which is pretty slow in comparison.
To test, install the Speedtest app and turn the Wi-Fi on. Run Speedtest and you should see transfer speeds close to the max you get over your internet connection (mine, for example gets about 10Mbps). Turn Wi-Fi off and try it again. You'll see whatever your 3G network provides which is likely in the 1Mbps range. Pretty slow.
So, if Wi-Fi is on and you run a file transfer like you want, you should be using the Wi-Fi connection and it should be pretty fast.
With wifi on yes you can both get phone calls and data like tmobile and at&t use data service and still recieve calls and text. Without wifi on cant be recieving.