I have both the SP4 i5 8GB and the SP4 M3. Testing back to back:
Comments relating to the M3:
Chrome - open loads of tabs, no issues.
Sony Movie Maker Premium - i5 is 60% faster to encode.
Hitman Absolution Game - 5-10fps different but actually doesn't really hamper the older games. The difference between the two was High on the i5 and Medium on the M3. The results of that is very little impact on enjoyment
Office - load all apps via O365 including Skype for business: perfectly responsive and performant.
Pen experience - no difference between i5 and M3
Fan - none in the M3 (lovely) but the i5 doesn't use the fan much but when it does it is really distracting.
Also memory: with 3.9GB used on the M3 it is still perfectly usable. Amazing chip really when you think about it!
Overall - keeping the M3
Just when I thought I had decided on the i5! If you're choosing to keep the m3 over the i5, I may consider it. The choice would be easy for me if the base i5 had more RAM / bigger SSD