As for performance, 95% of the time I don't care that I have the i5, and the other 5% of the time I remember to get up and do something other than stare at a computer.
Not wanting to spend almost $2000 again, I went with the base 13" retina this time (128gb ssd, 8gb ram).
I have almost 0 complaints. The screen is beautiful, it's significantly lighter then my 15 inch was and it's plenty fast. Photoshop / Illustrator have been fine when cutting up mockups. Xcode hasn't given me any problems either.
My only complaint is no built in ethernet jack (had to get the $30 lightning adaptor). This is obviously understandable given the thinness of the laptop, it just didn't occur to me it didn't come with it when I got home. Lack of CD / DVD drive hasn't bothered me yet.
Oh yeah - can't upgrade the ram either; had I known that, I might have gotten more (was panic'd and needed a laptop that weekend)
Storage space has never been a problem; my work is always on some server somewhere (dropbox, time capsule, a server, gmail, etc...) same reason i never get iPhones bigger then 16gb - most of my data, music, images are usually an app / website away
Mine is not a retina, so that could change things a little, but not much. The wider feel really matters to me, even on very high resolution screens. I've used the Retina displays, and a 13" has what it takes to get it done. Really, it's about overall feel, room to type, rest hands, whatever.
It's also about connectors for me. The larger models have room for a bit more, and I tend to connect a lot of stuff. If there are more / better connectors on the 15" (I've not looked), go for the 15, if having these options matter.
When I buy laptops, overall feel matters as I've mentioned, but core machine capability matters much more. I tend to run 'em for a few years, and once they are cycled out of full time use, run 'em some more. It's now to the point where I've cycled out all desktop machines, and that is possible by buying up on core machine specs and buying quality hardware that will run for a long time.
I would do the 15" for the better processor, assuming that i7 has respectable cache, and ideally the hardware virtualization modes included. I've never, ever regretted spending for the "sweet spot" best CPU. Production, day to day, machine life is extended by a year or so, and non-production life is extended several years.
I've an older Lenovo T60p that is still being used by somebody doing online school. It had the fastest Core DUO in it available at the time, and nearly 10 years later, that machine still works well on Windows 7. I put an SSD in that one a while back. Totally worth the extra few hundred back in the day.
Chances are, you may end up replacing the SSD over time, and that can always be cloned and upgraded when you need it too. If the size ends up being an issue, pick your time and score a huge one on a great deal, clone your existing one and go that route.
I would definitely go for the 15" given your use case and these choices.