I have just decided to go for the 13". The 15" is nice for sure, but for me it is just too much of a compromise. The 512GB will be so nice, I can just import stuff and not think twice and I'll never have to worry about using external storage or having to upgrade in a few years because I'm full. That alone makes the 13 worth it.
Plus it is the size I'm used to. It is really convenient and I can easily carry it and use it anywhere around the house. Also bringing it out once a week will be much easier and convenient. While the 15 is powerful and does have a nice big screen, the 13's screen is also beautiful and ultimately more convenient for me. I could see the 15 getting in the way at times, too. Additionally, I don't need the extra power as the 13 will already be a huge upgrade from my 2014 Air in processor, RAM, and SSD, and will be able to handle whatever I need to do. If I get to the point where I need quad core I can address it at that point in time; but I'm not gonna compromise on storage or convenience now for something I don't even need. Thanks for all the advice and input everyone! I will be getting a 13" 512GB.