I was very much on the fence about moving from my late 2013 15" i7 16GB RAM, 512GB SSD MBP to a 2018, as it seemed that the late '13 was handling my needs pretty well, and the 2018 pricing was/is $$$$.
The trackpad on the 2013 needed to be replaced, the screen coating was looking a bit tired, and the SSD was down to a little over 100GB (after cleaning-out) so I knew was going to have to put money into the machine if I was keeping the late 2013 (a lot less than buying a new 2018, though!). So the real question was: Do I put the money into the 2013 for another ~+/-3 years of life or a 2018?
Ultimately, I ordered a "trial" 2018, i7 1 TB, 32 GB RAM, so that I could experience it first-hand in my daily use, knowing I could return it within the free return period. The 32 GB of RAM was specified to give me a bit more room for an instance of Win 10 under Parallels that I frequently need to run.
I kept the 2018. While not a "stunning" upgrade over the later 2013, I was pleasantly surprised how all of the upgraded bits contributed to a smoother, somewhat faster experience, and actually found I liked the (now membrane-covered) keyboard on the 2018 better than the 2013's. Even the much-maligned touch bar has proven much more usable and convenient that I expected (I was anticipating hating it).
Note that the decision was made after my late 2013 MBP was repaired (at surprisingly low cost) into essentially "like new" condition -- new top/screen, new bottom case (body, trackpad, keyboard and battery), new bottom cover, and replacement headphone jack -- so I was really comparing a "new" later 2013 to a new 2018.
This all worked out well for my son, who just completed his Masters degree and wanted/needed a new machine for personal use (does classified work on his office machine; personal stuff on that is a no-no) -- he's now using the late 2013.