Sheeeeit, tough call. Need for Speed MW is probably my favorite racing game of all time, but Super Bomberman was a blast. My friend had it on his Wii, and we'd play that shit for hours.
On the other hand, Need For Speed MW was awesome. The graphics were pretty exquisite, and you had dozens of awesome cars to choose from. You could pick a Japanese tuner, a European sports car, or American muscle, and then you got to customize the car, from the engine to the spoiler, to the paint job and decals, to the rims and tires, you got to make that car yours. The racing was great–challenging but not overly difficult–and the police chases were outstanding. There was nothing quite like swerving around a corner, knocking over a pile of tires that leave the cops scrambling while you calmly cruise away.
I'm voting Need for Speed. It was a really fun game that provided weeks, if not months of entertainment. Super Bomberman was fun, but you could only play it for so long before it go dull.