The main reasons for playing number 1 first (since everyone is in agreement that the second is better) are story/continuity and difficulty. I don't care about video game stories at all (an SMB1 level of story is all I require) so the first one is not a concern. The latter is also not a problem as I will finish the game no matter how hard it is. The only reason I don't finish games is poor design, like I did with M&L: PIT and MP3:C.

i'd say if you can actually afford to get both, get them in order (pikmin first then pikmin 2). if you can only get one, get the second one. it's vastly superior.

don't play the second one first and then get the first one though, because then it will disappoint.

Just get 'em both.