I had the 2015 and the bought a 2017. there is a noticeable keyboard difference and the HZ rate is only 30 vs 60 with 2017 when plugged into a 4k ultra fine LG monitor. I still use the 2017 daily. I have the full size iMac Pro magic keyboard and swapping between that and the 2017 keyboard it doesn't bother me much although obviously the iMac Pro keyboard is top notch. the 2015 keys aren't really good. way too flat. go for 2017 or not at all
LG Ultrafine 4K (now) supports 60 Hz on the 2015, according to Apple:
Learn about the 23.7-inch LG UltraFine 4K Display and how to use it with your Mac or iPad.
support.apple.com
However, some people have complained the machine can sometimes feel sluggish when hooked up to a 4K monitor. I can't say myself though since I don't own the 2015. I have played with the 2015 a bit though (not with a 4K monitor).
The 2017 keyboard is noticeably better. The 2017 m3 is also noticeably faster than the 2015. The 2015 felt sluggish to me even back in 2015, which along with the bad keyboard is why I didn't buy the 2015. even though the form factor and overall design were what I had wanted for years. I waited for the 2016 and was still unimpressed, but the 2017 was a noticeable change.
Plus the 2017 also supports both hardware 4K 10-bit HDR HEVC decode and encode.
BTW, here is my MacBook performance thread:
I posted this originally in a different thread, but I think it deserves its own thread. See this post for results of the m3, i5, and i7, with the m3 tested on two different table tops...
forums.macrumors.com
This is a more complete graph of the 2017 models later on in that thread, that also includes a comparison of performance vs. table composition/insulating properties (since the machine is fanless):
As I suspected, the table material makes a difference. The first time I ran the test on my 2017 MacBook Core m3, I ran it on a granite counter, and turned off WiFi and Bluetooth. I just ran it again, but this time ran it on a wood table (which would act more as an insulator, keeping the heat...
forums.macrumors.com