Photoshop :: Best Specs To Run CS6 On MacBook Pro

I am considering a MBP for all my photoshop work. Currently using Photoshop CS6
 
From 2007-2010 I had my old MBP (late 2006 model). Then in 2011 I was using the 2011 iMac, 2.5ghz, 4gb ram. I sold the iMac as I felt more comfortble with the laptop in terms of working anywhere and anytime. Also for some reason I just strangly enough found the iMac a bit sluggish compared to the smooth performance I had with my old MBP. I mean the iMac is fast, way faster than my old MBP, However I do think that it may have been due to the wireless Bluetooth mouse that I was using.
 
I run psd files that have a large number of layers and can reach a size of 3-4gb per file while processing and when saved and closed upto 2gb. I rarely use filters, but when i do, its mainly the blur filters and then sometimes the others.
 
15"
I am considering the 15" MBP base model but bump up the RAM to 8gb... I wish it were more than 8 though... and getting a SSD drive of 128gb... too expensive other wise and this is enough for my work files as I have 2 additional backups on external HDs.and VRam of 512gb with an anti glare display.
 
13"
I like the size & portability of the 13", but the lack of a dedicated VRAM is putting me off. I could always get an external display later if the need be, however what about VRAM... its shared right? How will that affect CS6 performance?
 
I would like to really bump up the RAM but the only option i have is 8gb. Also I hope to use this system for another 3-4 years alteast if it does not have any other issues.