After testing MoI 3 with my wife's Surface Pro 3 (i5, 8 GB, 256 SSD) I decided to purchased a SP 4 a few days ago with the same specs (i5, 8 GB, 256 SSD). Both handle MoI well.

The SP 4 has a higher screen resolution, better pen resolution (4x) and better heat management so I think it is better for graphic work. If you use the touch pad or a mouse it is the same as any laptop to navigate but you can manage with just the pen. I loaded some of my average sized files with no issues thus far. I'm still learning the ins and outs of the SP 4 with the pen so sorry that this is a little light.

On a side note I can run blender with it but you have to set the font size to about 115 in the user preferences so that you can read the text.

For any of you using OnShape you can edit your sketches in the Chrome browser and see all of your lines (and in the correct color) rather than just a flat plane outline!

I will say the SP is a bit different to use but so far I'm not looking back to use my 6.9 lb Sony Vaio with a dedicated Graphics card.

Hope this helps,
David