[kbenson]

Here[1] is a break down of a core M and i5 mobile processor. I don't know if they are the specific processors, but there are some interesting differences, mostly around cache size, graphics chip, and graphics capabilities (the i5 has more hardware decode capabilities it seems). That said, since they are advertising 12 hours of battery, it's not like it's a slouch in that department.

1: http://ark.intel.com/compare/84672,85213

[revelation]

The most important part is TDP, 4.5W versus 15W. Now sure, both processors tick all the boxes, but at the end of the day, you get performance as you put watts in (certainly when you are comparing processors as similar as these two).

So the M will perform perfectly fine for YouTube, web browsing, office etc. but it will suffer significantly in benchmarks (and subsequently any high load / long time situations). Which is a very good tradeoff to make since you get 1/3rd of the power consumption but can run all the important usecases, if the office is what you're targeting.