Go for the 17inch MBP, the screen is big enough for everything you will want to do, Photography, musics, videos, coding etc... Here the only difference existing in iMac and MBP in my opinion is the size of the display and the prices. Regarding the performance of the MBP you can boost it by adding more RAM and 1T of HDD if you wish.
Opting for a MBP will allow you to be movable with your computer whenever wherever however you want. At the beginning you might think that it's too big, but once you get use to it you will never notice that is too big. Put it on a stable place in your room or office if you want to use as a desktop and your 17inch MBP will be just fine.
But opting for a MBP require you to have an eye on the battery and have it exercising some time if you wish to use it like a desktop.
I'm have this product which i use like a desktop and i never regretted buying it instead of the iMac, in fact even though most of the time i never move it around, times come when i need to carry my MBP somewhere to work with.
I read you are having a sony laptop so i could have suggested to you to go for the iMac, but since you are already having a Laptop, it means that you move around with it sometimes or most of the time, Just like the iPhone, i believe that after you will taste the OS X Lion for a month or so, you will not like to have anything to do with your Sony anymore, your MBP will then be able to follow you everywhere you wish compare to an iMac.
Hoo and be award of the 17inch MPB and iMac GUI font which in most of the native apps are very small to read. It's a pain for me to read the safari browser header, ical, iTunes, app store, etc... all are having very tiny fonts, and the more you try to make them bigger by changing the display resolution, the more blurry the screen becomes, everything else is perfect.
