no dGPU in 13.*"... modern converters do use GPU more and more (ACR/LR, C1, etc)... not that 15.*" has anything to write home about dGPU-wise, but still it can have it.
The Specs can never be the same, since all CPU's in the 15" is quadcore. And all in the 13" is dual core.
I have a late 13, 13" and I must say it is enough for my 12Mpix images, but as soon as I play with something like a a7r, i can tell it is actually struggling a bit. YMMV of course, depending on what you are expecting. However, that is the only time I feel it is a bit underpowered. I got the base i5, with 128 ssd and 8 Gig of ram.