I have both a Surface Pro 3 (i5, 256/8) and a Lenovo X1 Carbon. They're both great machines but frankly the SP3 is the best device I've ever owned. I've never had any overheating problems and I take it with me everywhere. I use it as my personal tablet when I need a tablet, it replaced my X1 Carbon as my work laptop (my X1 just collects dust now), and I'm a photographer on the side so I run LR/PS on it as well as use my Intuos 4 with it.
So if the Core M improves performance of the Surface Pro line while making the battery last even longer, then I'll be first in line to get an SP4, assuming it has the M.