In the past, I've always picked characters based on how much I actually like them as characters, and if I like the way they "feel".
I love Yoshi, but don't play as him because I don't like his playstyle.
I like the way Captain Falcon plays, but I don't like him as a character. So I don't play him.
For Melee, that mostly meant playing as Falco, Ness, Pikachu, and Fox.
For Brawl, I haven't pinned down any mains because no matter how much I like a character, I won't play as them if I don't find them fun. Sonic is one of my all-time favorite video game characters, and judging by the demo vids and general comments from those who went to E for All (specifically Gimpy), I have very little doubt that I'll be disappointed with him. But I still can't call him my main yet.