this is always a tough one, i used to say CD's for sure, but you don't have to put your songs on your mates laptop, you can just run them off the usb, traktor is pretty quick at analysing the tracks (this also depends on the usb port as well eg usb 2.0 usb 3.0 etc etc) but i would do both, when dflux came to mine a couple of weeks ago he picked up traktor faster than me, and he was mixing so quick off it, its designed to be a natural progression, and if your mate uses it he will be able to show you pretty quick, but if your worried, just use CD's