What should I get?

So I come in here for a little help for everyone

I'm looking to buy an entry level Macintosh, right now Im just stuck between the 13 inch MacBook Air, the 13 inch MacBook Pro or the 21 inch entry level iMac.

Right now, I'm just torn between them, I'm leaning towards a MacBook Por or Air because of the portability and being able to have performance on the go, but the iMac offers much more performance but I'd have to stay stationary, even though I would have more room to work with with that 21 inch screen, while the Air is teasing me with its 11 hour battery life and solid performance.

To be honest I'm slightly leaning for the MBA since I'm not running around with a tablet yet, I guess I'm just looking for a little persuading.

Anyway, any form of feedback would be appreciated to help me with this purchase.