I was recently looking at both but with the understanding I already have an iPad mini and I was replacing a Chromebook. I played with the SP3 at a couple places and just never felt it to be a laptop replacement, although a very nice tablet replacement if one doesn't mind its size vs. ipad mini.This goes a long way as well. I prefer OSX, but to be honest, I work in windows and I use Office more then anything so a windows machine in my case as certain advantages even though my proclivity is towards OSX
I like to use my device in my lap so the SP3 just wouldn't meet my needs. It's not often I put a laptop on a desktop surface so flexibility in seating position was mandatory. I'm not a fan of touch if I'm being productive so having a touch interface also made the SP3 less desirable. I'm much more efficient with a trackpad than with a touch interface and it limits my arm motions when much of what I'm doing is on a keyboard. I chose the 4GB/128GB 13" Macbook Air and couldn't be happier. Amazing device for sure and a 12" screen in the upcoming model isn't desirable to me since I was moving from a smaller 11.6" Chromebook and wanted more real estate space (size) for my 40+ year old eyes.