Apart from the odd touring company (the TNT theatre Britain, for example), there's not much on in English in Zurich apart from the ZCC - never seen any of their productions, so I can't comment on them, I'm afraid.
There is however quite an active amateur & semi-professional English-speaking theatre scene in Basel, with three main groups:
Semi-Circle (
www.semi-circle.ch) - An amateur theatre group who usually put on two productions a year. Last one was RENT, which was spectacularly good.
The Gay Beggars (
http://engsem.unibas.ch/department/p...e-gay-beggars/) - the theatre company of the English Seminar at the University. The 'Beggars' have been around for quite some time and do seriously good plays each Spring, usually around April/May. Next years' is 'Noises off' by Michael Frayn.
Upstart Entertainment (
www.upstart-entertainment.ch) - A production company who do English productions with local talent. Used to do mainly musicals, but they recently branched out to plays (last one was 'Doubt'). Their current show (premieres tomorrow, actually

) is an openair performance of A Midsummer Night's Dream in one of the medieval courtyards of the University (
www.shakespeare-basel.ch)
Also, the Theater Basel has an English season ticket for six shows (from British touring companies).
http://www.theater-basel.ch/englishabo/
Basel's only 55m away...