Luigi, and here's why.
When they debuted, both of these characters were pointless in existence other than to allow a second player. Tails was more credible, at least, because Luigi was (I think) just a pallette swap and therefore a Mario clone.
However, Luigi was the first to get his own game.
Furthermore, up until just recently, playing as Tails has been pretty much pointless, because he had nothing to make him a better character than Sonic. Unless of course you count helicoptering around after dying, which he spent most of his time doing.
Luigi, on the other hand, has been a markedly different character than Mario since all the way back to SMB2. This trend continued in all the RPG-style Mario games, of which there are now quite a few.
Tails, on the other hand, did not really need to exist until the Sonic Adventure games came out and actually gave him his *own* playing style.
So yeah.
Go Green.
