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.
No - High Sierra isn’t generally slower than Sierra so there’s likely something else going on.
I like your suggestion to run a fresh install. That’s almost always the fix when a system gets slower and slower and even a fresh install of the same OS as you have running for 6 months to 2 years straight often makes everything snappy and you happy again.
I’d do one last backup and erase install High Sierra and migrate the data back. Take some timings so you know objectively how long a boot takes and how long the things you think are repeatable (opening 5 tabs in Safari / opening a large spreadsheet / etc...) so you can then dig in if the reinstall doesn’t make everything better both subjectively and objectively.