While I don't use multiform characters very often, but..
Samus/Zamus: Same reason as Shadow's, I play Zamus because my Samus is about as terrible as a 60 year old geezer who's never played a video game because he has arthritis picking up smash bros. for the first time and chooses Jigglypuff while having a 300% handicap on his Jigglypuff while having a team battle against 3 lv. 9 Ike's. ...Yeah. DX
On a more serious note, even if I'm terrible with Samus, I hate projectile characters that have only that for their specials(or mostly). Samus is more of a wait while throwing projectiles, dodge incoming approach, counter enemy away, fire away projectiles again and repeat. Even then, her offensive manuevers are very generic.
While Zamus is a combo beast and also quite quick. She has a bit of priority in some moves, very unique stun moves and just original moves in general, and even has long reach in some of her moves. Zamus' playstyle can also be much more confronting and physically approaching the enemy, which can be either passive or aggresive - aggresive being my very nature. And as mentioned before, she's very unique which I like in a character.
Zelda/Sheik: I used to play Zelda better and much more, but as of late my Zelda is.. bad. So, I play Sheik. Zelda is a bit slow and her offensives are very generic. While she's better than Sheik as her moves are very powerful in both strength and usefulness, I simply don't like that in a character. She's also very spammy, and I try to avoid that as much as possible - her strategies can be very rinse and repeat without becoming very good with her, which I won't spend time doing.
Sheik on the other hand, is faster and is insanely comboable, perhaps the most combo-crazy character in Brawl. Her moves are also very quick at the same time, and racking up damage is no problem for her. Some of her moves do have slightly above average range too, and most of them have good range anyways. While she lacks my favorited, aggresive KO power, I can still play somewhat ably as her. It's not like I sub her or anything, anyways.
Don't play Pkmn trainer.. Don't know much about the pokes either. >.>;