I thought i should jump in here.
I have just sold my 2010 iMac 21. Was a lovely machine etc massive screen looks the nuts and everyone comments. BUT it wasn't portable. I dont mean portable like a MBA i just mean i couldn't pick it up and take it to my mums house or sit in the garden etc etc.
So anyway - i deliberated for quite a while, 11" MBA, 13" MBA, 13, MBP, 15" MBP.
In the end i opted for the 13" MBP and heres why:
I can upgrade the HD to an SSD and upgrade the RAM, thus making it faster than the MBA (ok not as slim but seriously this thing is hardly a brick!)
It really is a perfect size. I use Photoshop, iTunes, App Store, Google Chrome, Microsoft Office almost extensively and i have yet to find the screen to small. The 11" would of been this is a happy medium.
I dont have the bulk of the 15". Yes the 15" is portable but its still big and heavy.
The sound is PLENTY loud enough for a laptop. Personally the sound of a laptop should be loud enough for me to hear it not loud enough for the neighbors. If i wanted to blast music id buy something that blasts music. I expect my Mac to be able to operate PC functions - not blow out my windows.
I have the option to connect a screen or 2 to view a large display if i want to. By having the 13" i now have an ultra portable machine with the ability to have 2 27" screens side by side. The 15" doesn't have any advantage for this. They can both do it so i might as well have the smaller lighter machine to lug.
The resolution ... sigh ... what is the problem? This screen is higher than many i have seen before? Sure its not top of the mac league. But i bought a base model MBP i didnt pay £1000 more for the highest end. Did i expect to be able to spot wally? Not really. Does it look sharp, crisp with good colour? Yes. Stop moaning.
All in all £859 later (student discount) its an awesome machine and i couldn't be more thrilled that i have one!
Only thing is now im lazy and CBA to plug in my time machine every day so i need to get a proper Time Capsule