Just from reading the specs of the M3, I think it's going to perform like the i3 of the SP3, or perhaps better, but only when it really needs to (or maybe a better phrase is "only when it can"). The difference between the Core M series and the i5/i3s in Surface Pros is how it boosts. The i5 is going to stay at constant clocks under load assuming it's cooled effectively, where as the Core M3 is much like an i5 or i3 that's been given an extremely strict set of rules regarding how much power it can use and how hot it can get.

The PC/tablet OEMs set these power/thermal targets, so it can vary from unit to unit. Not that the Core i chips aren't subject to the same set of limitations, but they are spec'd for devices with active cooling, hence the higher base clock, which is a more important number than the boost clock. I believe the M3 SP4 is going to be passively cooled, so the jury's still out on how the chip will perform (plus, the M3s are new. Only last-gen core M's have been field tested and benchmarked so far).

To be honest, the price difference between the M3/4GB/128GB and i5/4GB/128GB is so small that I'd just go for the i5 unless the slightly longer battery and passive cooling is an attractive feature and your use won't extend beyond browsing, tablet games, netflix/youtube, and Office. It's odd to see it in the Surface Pro, a machine meant for real productivity. If I were buying a SP4, I'd get the i5/8GB/256GB -- well, if the customization page would let me, I'd drop down to the 128GB SSD to save some dollars. (Seriously, the customization is a joke. Can't get a bigger SSD if you only get 4GB of RAM? Only the 256GB SSD is available with the i7 unless you get 16GB of RAM, yet the 512GB is available with the i5? And why is it $400 to go from 256GB to 512GB?)