Mariners have an all-time .470 winning pct vs a .481 for Milwaukee. That means Mariners, on average, go 76-86, Brewers 78-84. Mariners have a four-season stretch where they went an average of 98-64, 14 winning seasons out of 43, compared to Brewers 17 of 51, they have five hall-of-famers largely associated with their franchise (possibly 2 more with Ichiro and King Felix). I wouldn't in any way say that Milwaukee has had "significantly" more success, unless you are weighing recent results more heavily. Milwaukee has a pennant, more division titles (assuming you count 1981), more post-season appearances, and as you pointed out, a higher overall winning percentage. I think it's significant.