1) Portable game is portable. It's perfectly suited to a few minutes or a few hours at a time. You can stop in the middle of a battle and resume really easily, too. Tactical RPGs are good for that. That's why there's so many of them on handhelds. Actual mission length varies, but it's so easy to suspend your game and pick it up later that it really doesn't matter how long they are.
2) Talk to people with Chrom to recruit them.
Don't accidentally kill Gaius like I did.
3) Well, you know. There's a lot of SpotPass content and DLC, plus StreetPass functionality, so it's a pretty convenient game to have on your system at all times. On the other hand, there's no price difference and you can't lend/resell it. Don't worry about it too much. Just get whichever version you want.