I am anything but biased towards Mario. I was not excited at all for NSMB Wii. I wanted it to fail.
But I love this game. I love it more every time I play it. Yesterday I accidentally stumbled upon a secret path in World 7 that lead to a completely awesome level I'd never seen before. A game hasn't given me such a feeling of accomplishment in a long time. The level design and deceptively deep mechanics are just brilliant.
I haven't played Splosion Man but NSMB Wii deserves all the praise it gets.