Bigger...is a difficult thing to judge as it really depends what you measure. One of the problems is that often different markets come into play.For example Pearl Jam had a massive impact on the US market but not on the UK one(as did Nirvana). The U.S does seem to plough its own little furrow;the likes of Journey,Foreigner and a host of other 80's style US bands were largely ignored by the UK.
If you were to judge bigger as album sales then Pearl jam would win hands down.
If you were to judge bigger on popularity right now then MS would win hands down.
My feeling is that MS is a trend that will lack longevity and certainly they haven't had anyway near the impact that PJ had when they first burst onto the scene.PJ's arrival changed the music landscape forever.
Mike