Macbook Pro 15" vs 13" - Performance vs portability
Solution 1:
Here's the way it goes in our household:
I own the 15" Model you describe above (albeit a slightly earlier version with the GTX750M GFX card) and my other half has the 13" you describe above (again, albeit 6 months old at this point)
Now, she does a lot of graphical work on it in Adobe CC - Photoshop, Illustrator, InDesign, etc - much the same as you want to. She says it never skips a beat for her, even when she's running it on external displays in the office and driving the graphics side of things hard. She sometimes gets frustrated with the lack of screen real estate compared to mine (when she does use it) but it's generally not a problem for her. The battery also performs a bit better, which helps if you're on the move a lot. Sure, a bit of extra power would help her but the portability and price trade offs aren't worth it.
Based on what you've said, I'd recommend the 13". It's plenty powerful for what you want, but I would use the left over cash to buy a couple of nice 24" external displays if you have the space to set up a small office. I love having 3 displays running (1 on the MacBook + 2 external.) Better on so many levels!
Another note - it used to be possible to upgrade MacBooks (more RAM, swap out the HDD for an SSD, etc) so I used to recommend buying less and adding to it when you need it. Can't do that anymore, so buy as high spec as you can afford.
Solution 2:
Personally, the GPU is probably important for what you mention, so I'd consider something entirely different.
You wouldn't go wrong with the SSD storage in either machine and the CPU are both so fast, you won't be realistically waiting for either. I personally find no problem using the 12 inch MacBook Pro for heavy graphics, image processing, Xcode and such. Portability is more important for me and I didn't know about the iPad Pro when I bought it.
I'd get the iPad Pro and not look back. If you decide you absolutely need a Mac too, spend the rest of your $$ on the 4k 21 inch iMac which has far more bang for the buck in terms of CPU/GPU than any portable and you're basically getting a free amazing screen. Even if you bump the RAM to 16 GB and spec SSD storage on that, it's the same cost as portables you mentioned.
I've spent three separate 30 minute sessions with the Pro iPad and the keyboard and everything about it is far superior to my MacBook 2015 and I choose that over either the MacBook Pro you have recommended. It's smaller, faster for what I do and less cost. I have access to a 27 inch iMac retina for some graphics work, but mostly just use the MacBook connected to a nice display. I'm selling my other iPads and will be getting an iPad Pro since drawing on it is superior to anything anywhere I've seen. No cintiq, tablet, mouse can hold a candle to the Apple Pencil and the software on it for drawing is going to be so good so fast, my feeling is almost everyone will feel hamstring on Photoshop and Illustrator on the Mac in a few months time.
If you're not game for that, my opinion would be the 13 inch is more than enough for the task unless your billable rate is such that you can recover the investment in made up time or you really need the screen size to take with you.