The surface pro 3 is a 12" screen, it has more horsepower under the hood and is about 400 dollars more expensive than a comparable surface 3. The surface 3 is 10" but still retains the 3:2 form factor which is one of the main reasons we want them, the 16:9 form factor of our asus tablets don't work well with next issue and what not. Plus, going to the surface I get full w10. Not stuck on Win 8 RT. Not that RT is bad, its better than both android and IOS tablets by a mile. Just I have a couple of .EXE programs I want on my tablet. The surface 3's fit our needs perfect. My wife only needs 64 gb of storage and I need 128gb for programs but use a USB drive for storage of files, photos, media and what not. I am running 120gb drives in my notebooks with no issues.
I was going to go with the 256gb Surface pro 3, but the bigger tablet, and double the price is not worth it IMO.
The 128gb surface 3 here is 739, the surface pro 3 with 256gb is 1599. That extra space and horse power is not worth it to me. Especially when I am buying 2. I can outfit my wife and I with 2 tablets, type covers, pens, and a docking station for a little more than the cost of the 256gb and type cover.