I personally would rather pick the Michailovsky. The quantity of premiers within recent couple of years, the European and Russian stars who have worked with it, the wide opera repertoire - all of this keeps the local public interested and excited. I would say that Michailovsky is fashionable in St.Petersburg - in a good sense of this word.
The Mariinsky is an old, traditional brand. Top quality for a top price. A no risk choice.
Also please have a look at my comments regarding the Dress code for Ballet & Opera performances.