I'm a foreign festival virgin, and I'm deciding which one to go to out of these!
Being from Manchester, I'm sure a the Mancs would opt for EE and it would be good to see familiar faces, but I've always fancied Sonar too!
Also, what's the crack with Off Sonar - is it just a series of other parties around the city at the same time?
off sonar is just as you say. a lot of parties at various locations, some great and some not so great.
The main sonar event is good (although the night event is enormous and has no real intimacy at all) but you will find equally as good dj's outside of it.
I went to the Garden Festival in 2012 (same site, 1 week before EE) and i highly recommend it if you want to chill out as well as dance. The site is beautiful and the location stunning, no chance of being disappointed by a poor off sonar party that gets shut down.
I would plump for EE.