I have Late 2013 13" rMBP. I'm not going to update for at least 2-3 more years - this machine is very good. I wish I have bought 15" though.
Going back to topic - if I was going to buy 15" rMBP today, I'd certainly go with 2015 version. MagSafe and SD card reader are essential to me.
Bad battery and GPU reputations don't help. If I'm spending so much money, I need to be 100% confident that I'm buying good stuff. Right now there's a risk that those two components have engineering defects.
It's also worth to mention how little did rMBP change over those years. This Intel CPU is barely faster than Haswell. Battery life has not improved but has rather deteriorated. The SSD is faster but it already has been blazingly fast in 2013 - you cannot notice any difference in day to day use. The keyboard is not better it is just thinner.
The only aspects that have really improved are display and trackpad. The display is brigher and the trackpad is larger. It's the same notebook just a little better.
Some of the improvements are small, as you say, but the new MBP 15 is still clearly better than the 2015, not a close call. A summary of the differences:
The new MBP 15" is clearly better than the 2015 model in:
-- Screen (brighter, better contrast, better color)
-- dGPU (early issues have been fixed already)
-- SSD speed (it is significantly faster)
-- External monitor support
-- Heat
-- Quietude
-- Speakers
-- Touch ID
-- Touch bar
-- Size/weight
It's also mostly better in regard to:
-- Battery life (better for ordinary use, as shown by professional reviews)
Mixed:
-- Ports (the new machine has by far the more powerful and flexible ports, but the 2015 has by far the more convenient ones for most people not yet living in the future)
-- Keyboard and trackpad (the new ones get mixed reviews compared to the old)
The one area in which the 2015 model has a clear, unmixed advantage is:
-- Replaceable/upgradable SSD (and if the motherboard fails, you won't need Apple's help to recover your data)
A better MagSafe would be nice, but the old one was a lot of trouble for a lot of users because of poor cable covering and dodgy connections.