Wow, pretty surprised that
nobody has managed to post the correct information for the core series differences from i3-i5-i7 lol. So much misinformation in the last dozen or so posts
The difference breakdown works like this (Assuming Mobile/ULV parts)
Jumping from Pentium (I know there isn't a Surface model with a Pentium, just using as reference) to i3 gets you Hyperthreading, an extra meg of cache (50% increase) and the newer series of HD graphics. This is actually the most substantial performance bump you'll see in Intel's sets; You typically see a 100% performance boost when comparing a Pentium model to it's i3 counterpart.
Jumping from i3 to i5 gets you Turboboost for the CPU and a slightly higher dynamic max clock speed for the GPU. You get all of what would be considered the "business user" extras, like vPro and SIPP. You also get features like Smart Response, but that only comes into play when you have a mechanical drive, so it won't affect the Surface Pro series. You generally see a 40% performance boost with an i5 compared to its i3 version, mostly due to Turboboost.
The i5 to i7 jump is much less drastic than the others when you're dealing with ULV/Mobile parts. You are still on the same 2core/4thread system as the i3/i5 sets. What you do get is an extra meg of cache (33% increase over i3/i5) and generally higher clock speeds. The difference here is, truthfully, negligible when you consider the price difference. You're usually looking at a 15-20% performance boost over the i5 it goes against, which while decent, isn't worth the hundreds of dollars it will cost. And to make the i7 jump even less satisfying, Haswell i7 parts have proven to be less power efficient than the i5's, meaning you'll actually take a slight (Usually 5-10%) hit to your every day battery life in comparison.
Of course, we still don't know what i7 part MS is using. They could surprise us all and bust out an HD5000+ chip, which would make the i7 option much more appetizing considering the huge GPU performance boost over what the i5 would have.
All of this information changes depending on what kinds of parts you're looking at, by the way. It's completely different with desktop parts, and gets most confusing the Mobile laptop option parts. The ULV sets that the Surface Pro uses (And the Y series that they'll probably move to in the next iteration) are pretty straight forward however.