If I had the cash, I'd go for the MMr just because I just love the theme, the artwork, the music and the humor.
As a GOTPre owner, I'd say that it's a much faster and better game than the Pro (I tried both extensively). To me, the Pro's layout and gameplay are boring. With the Pre, at least 70% of the shots are on the lower half of the lower playfield, and down there the action is fast. And if you want to mix it up a bit, you can bang on the upper playfield. But whether you get the Pre or Pro, you get an incredibly deep ruleset with numerous strategies you can use to achieve different ends.