No matter whether you choose the i5 or i7, you're going to get great performance. I throw everything at my Surface Pro 2. It is my primary machine. I use my SP2 for EVERYTHING. There is no case use I don't have for my SP2 except for torrent downloading. Sure, it's not as powerful as my desktop, but my SP2 is a beast. I do a lot of Adobe InDesign, Illustrator, Photoshop, and Premiere Pro. It's all amazing with my i5/8GB/256GB and my i5/8GB/512GB machines. The best is that it comes with the Wacom pen (note that SP4 uses Microsoft's N-Trig technology) which is truly useful for any graphic design work. If any of those programs had any difficulty, it's Premiere Pro. But if I need to do some video work in a pinch, I don't mind doing it on the SP2.
If it's between i5/16GB/ and i7/8GB, I would go with the i7 for the graphics. The Skylake processor line is better about scaling frequency than previous generations so if your workflow is more i5 level, you will get equivalent battery performance to i5. However, since the i7 can scale higher, if your workload is heavy, your battery life will be worse than the i5. 8GB is terrific for the near future. You will see and feel the benefits of the i7 long before you feel the benefits of 16GB unless you're editing 4K video. 4K video will thrash in 8 GB of RAM. Then again, the HD 520 is a little slow with editing 4K video anyway. Not so much that it's unbearable but it will be sluggish.
Iris graphics are not a panacea (if you want amazing graphics, you have to get away from Intel), but it's a significant step up from the HD 520.
The biggest factor with battery life on the SP4 is still Windows 10 and its buggy power management but it should get better over time. Edge and Cortana are also factors. Turn off "always listen" since you can always click on the Cortana button or click the pen button to launch Cortana. Edge really needs some work on both a performance level and a features level. It's pretty fast but there are times that it causes the PC to run hard for no discernable reason. There's the Fall Update which is dropping today for Windows 10. The next major update is Redstone which is set for next year. And of course, incremental patches and updates on a weekly or monthly basis.
Surface Pro 4:
6th Gen 2.2-GHz Intel Core i7-6650U processor with Intel Iris graphics 540
6th Gen 2.4-GHz Intel Core i5-6300U processor with Intel HD graphics 520
6th Gen 900-MHz Intel Core m3-6Y30 processor with Intel HD graphics 515
Surface Book:
6th Gen 2.6-GHz Intel Core i7-6600U processor with Intel HD graphics 520
6th Gen 2.4-GHz Intel Core i5-6300U processor with Intel HD graphics 520
Note: SB also has the discrete Nvidia GPU as an option. So the Intel graphics is in the tablet portion when you undock. The Nvidia takes over when docked but it's not that simple. To save battery, the Nvidia isn't always running. The PC will actually switch back and forth between the Intel and Nvidia depending on case usage. You can manually force it to only use Nvidia or only use Intel.