IMO, you aren't going to see anything really exciting in any of them. Magnolia carries the highest end stuff of the 3, but Fry's has the most variety of stuff that you may or may not need...(toasters, washing machines, etc...???)
Where are you going to be? Cuz I'd hit up some real shops if you want to see good stuff.
If you get a chance and want to see some really nice stuff, head over to
Music Lovers in Berkeley - just a few min from SF.