I sometimes feel like I made a mistake in buying the base 15". I fell into the trap of thinking I need 16gb of ram in my portable to run illustrator, photoshop, and some web dev apps (brackets, sass, atom), and for what I do on the go 8 would be plenty, and the 13" more portable.
Ultimately it depends on what you're doing. I still have 2 days to make up my mind on the 15"/13". I'm honestly not sure what to do.
But unless you're doing a lot of heavy lifting, I'd say 8gb is probably enough, only my opinion based on my usage.
If you don't need the 15" I would get the 13" it's cheaper for a start. I'm looking at the 13" because I've seen it in person and it's nice and compact to carry around but also not too small that I couldn't work on it. The most apps I use would be Pixelmator, Photos app Safari, Pages and word processing apps as I write a lot.