Which should I buy, 13 inch or 15 inch (Late 2013) for Graphic Design?

Question

Hi! I'm in the market for a new laptop and after doing research for a month+ I've decided to go with a MacBook Pro (my first). The thing is, even after all my research (I've practically scoured the whole web), I can't figure out whether it's better for me to get the mid spec (2.4ghz/8gb/256gb) 13 inch rMBP, the low end 15 inch rmbp (2.0ghz/8gb/256gb), or the top spec 15 inch rMBP (2.3ghz/16gb/512gb). If I get the mid spec 13 inch or the low end 15 inch, I  will spring for the 16gb upgrade.
I was wondering if I could get the community's advice as to what is best suited for my needs. I'm thinking the 13 inch will be okay for me but I'm worried about the GPU not being able to handle what I need to do. I'm willing to spend the money for the $2599 model if I must, but if I don't need to spend that much on it, then I'd be happier. I don't want to spend on a laptop that's way more than what I need.
256gb space is more than enough for me, so ssd size is not a consideration.
Here is how I'll use it:
- (Most probably multitasking these all at the same time)
- Skype/Facetime/Instant Messengers
- Web browsing (most commonly with around 30-40 tabs at a time)
- Heavy Adobe Photoshop-10 hours 5 days a week (sometimes I also use Illustrator and Flash)
- Video editing here and there (maybe once or twice weekly)
- Streaming video/music OR playing music/video via iTunes or VLC
- Some downloading here and there
- Rarely, I'll also use it for word processing
I do a little gaming here and there, but most likely I'll only be playing Diablo III, Path of Exile, and Sims 3 (or 4 when it comes out), so I do need a computer that can handle it.
Considerations:
- I work at home so I'm not too concerned with the lessened portability of the 15 inch. I DO NOT want an iMac, please do not suggest that to me!
- I'm on my computer almost 18 hours a day and will need a computer that can keep up
- I'd like for the laptop to last me at least 4 years, since it's a big spend

Answer 1

Thanks! What I do want to know however is if the tasks I need to do can be handled without the dedicated video card? Also since I'm multitasking, is it better if I go for the quadcore?
I wish you could put a video card in the 13 inch. :/