Are you more into waves or freestyle?
Wind: definitely Mauritius, more consistent and stronger. I was there for 2 weeks in June/July this year and we had enough wind to kite EVERY DAY (we skipped 2 days). I mostly used my 5m-7m with surfboard. I spent 10 days in Zanzibar in 2014 and we got wind 50-60% of the time. Mostly 10m weather.
Flat/waves: for waves go to Mauritius. The waves at the outer reef in Zanzibar are really small. But if you are regular like me you will still have fun, only downwinder I have ever done on my good size, so yeah there is that

. For freestyle, the lagoon in Zanzibar is really flat on low tide (which is amazing!!) but choppy as hell on high tide.
I recommend both places