#1
What are the differences between them besides wattage and would the 15 be good enough for just my room?
#2
http://www.peavey.com/products/Vypyr/index.cfm

Says the differences between the 15 and the others.

15 would be enough for just your room yes but don't count on it for anything more than just practice in your room. in short the 30 gives the option to use a footpedal and gives you some more effects and looper functionality. if you can afford it i say get the 30 because it gives you some extra options. the one thing that I feel is missing in the 30 is the USB interface :-(