I personally wouldn't get it. They are different games, but have very similar mechanics. You are just substituting Nintendo characters for the ability to customize.
As for myself, I only need one great game per genre (BRAWL is my fighting game, Burnout is my racing game, Halo is my shooter). When I have two very similar games, I tend to stick with the one that's better overall. Why play a similar, somewhat inferior game when you have something better right next to it on the shelf? This is why I already traded in No More Heroes 2. I find the first one much more fun to play, even though NMH2 does improve on some things. But I just knew that anytime I ever get a hankering for NMH, I'm gonna just pop in the original. One of the few exceptions to this rule is Super Mario Galaxy and SMG2. I couldn't part with either of those.
So in my opinion, in this scenario, I would say that it's better to play the game first (borrow or rent) then decide if it's worth your time and money.
*also: They just put up a review of MNR on Gametrailers.com. I suggest checking that out.
