Keep in mind that the Surface Book will actually have two batteries to potentially deal with...one in the tablet/screen part (i.e. used when the screen is detached to operate in tablet mode) and one in the keyboard part (i.e. for when used as an actual laptop).
If you want, here is iFixit's "teardown" of the Surface Book. It should give you an idea how accessible the batteries are, which looks to be "not very" (i.e. requiring major "surgery"):https://www.ifixit.com/Teardown/Microsoft+Surface+Book+Teardown/51972
Note: This is NOT a repair guide, but a "teardown" that has the sole purpose of complete disassembly to see what parts are included, but also get an idea of how repairable it might be.
Step 30 of the teardown does say this under one of the bullet points:
After the difficult opening procedure, the SSD can be replaced. So too the glued battery in the display. However, the base battery is very heavily glued.
So, it appears that the battery in the keyboard base (i.e. the main battery) is not soldered down, but it is heavily glued down. And then there is the challenge in just cracking open the keyboard based (and the display) just to get to the battery. So, it appears that you could replace the battery yourself if you really wanted, it is NOT a task for the faint of heart. It appears that iFixit does give the Surface Book a "repairability score" of 1 out of 10.
As to a comparison between the two, they are kind of two different animals unless you equip them similarly.
The Lenovo P70 is a mobile workstation that is aimed at people who need serious power, especially for business related graphically intensive tasks (i.e. thing like CAD and 3D modeling) and where weight is less of an issue (it weighs at least 7.5 lbs). As such, even its cheapest "base" option comes with an i7 Quad core processor and a Quadro workstation oriented dedicated graphics card.
The Surfacebook is more of a business/prosumer level 2-in-1 laptop that is meant to rather light (about 3.5 lbs)and portable, yet still be able to pack a punch if needed. As a result, its based model (which is still only about $300 cheaper than the Lenovo P70's base model) comes with an i5 processor and only integrated graphics.
Now, you can spec them similarly (i.e. both with i7 processors, although I cannot fully tell from Microsoft's page which i7 it is and I am too lazy at the moment to look elsewhere for that info; and both with a 256 GB SSD) and their price gets pretty close (both around $2100 assuming no other changes other than switching the Surfacebook to an i7, which also gets you a discrete graphics card, and the P70 from a 500 GB hard drive to a 256 GB SSD). But, even then, they are still dramatically different. The Surfacebook will still be way lighter and more portable while the P70 will have a larger screen and a workstation graphics system that is more aimed to things like CAD and 3D modeling than gaming.
So, to me, you need to first determine what you really need and/or are after. From your description, you likely can live with a Surfacebook with an i5 processor and only integrated graphics (although what kind of software development could effect this). If so, that means you can go with the base Surfacebook model and thus get the lower cost. Then the question is whether or not you can live with a 13" screen as opposed to a 17" screen, which from your post sounds like you can. The last question is whether or not you need or want an easily removable battery. The Surfacebook's definitely does not meet that criteria, while the P70's apparently does.
So, to me, the question boils down to if you are just deciding between the two...do you want a light, highly portable 2-in-1 one with a 13" screen and a hard to replace battery and that can be bought for a lower price but with no discrete graphics or a heavy mobile workstation with a 17" screen, an easy to remove and replace battery that will definitely come with a discrete graphics system, but will have a 500 GB hard drive rather than an SSD in the base model (thus requiring spending more if you want an SSD)? After you answer that question, you can then decide if there are any upgrades that you want to do beyond the base model.
Even for me personally, it is not a clear cut choice. It depends on what need I would be filling. Since I don't really travel these days and thus don't need portability, I would personally would likely pick the P70 of the two. But, then I am not in the market for a Window laptop, or even a laptop in general, so I don't really have a need.