I use my 16G Wifi+3G unit primarily as a 30-volume Fakebook, with over 7000 titles. Once the new OS is out and I can multi-task, it will also play the bass lines and orchestral backing tracks for my solo piano gig. I also have the Petersen Strobe Tuner onboard (and carry a tuning hammer.) Plus iTablapro, Amazing SlowDowner and iRealBook; together I have a VERY large-scale song selection capability (I know a few thousand by heart, but the iRealbook backing track feature really opens up the world!
For my PDA needs, I use the 4G Touch, I'm NOT going to lug that big puppy around unless I'm on the road. So it's primarily a pro tool (no pun.) I also will use it for watching videos and web browsingI keep a few games on it, but mostly use the Touch for that.