Not huge, assuming the GPU in each is the same. Slightly higher base and turbo clock and a bit more cache.mi think someone said it was the i7-6560u in the surface book? Don't know what i5 they'll use
Best Buy is listing the the 6300U for the I5 model and the 6600U for the I7.
http://www.bestbuy.com/site/microso...lver/4531200.p?id=1219758515832&skuId=4531200
http://www.bestbuy.com/site/microso...lver/4531300.p?id=1219758514591&skuId=4531300