If you want a game that lasts, get KI:U.
That game is huge in every possible way, just take the soundtrack for example, it will take you several hours to listen through the whole thing!
If there's a game you absolutely need to prioritize, it's Kid Icarus.
About the controls.
If you keep playing it will quickly become second nature. Don't let the reviews scare you off.
Most reviewers haven't played the game for more than an hour or so, plus they just like to bitch about it since it isn't dual analog.
When you play it you either use a thumb stylus or hold your 3DS properly, I seriously can't stand the way reviewers hold it, they hold it like if it were some fucking balance act, really!
Do they purposely want to have cramped hands?
You're supposed to relax and straighten out your left hand fingers in order to divide the weight of the system properly!
Bottom line, they're both worth owning in my opinion but given the choice I would undoubtedly go with KI.