I think among Apple devices running IOS , the biggest beneficiary from the upgrade to IOS 5 is the IPAD . In IOS 5 now you get:
Notification bar(on all IOS devices) , then the split keyboard and docking(Ipad), five finger swipe upward and inward(ipad) , camera icon on lock screen(iphone) , Safari reader( all ios devices), revamped music interface(ipad). Other things either i dont know or i dont use. but if you want to update only 1 device to iOS 5 at a time , definitely update the ipad first , then iphone , then ipod touch.