Neither is a good option at the moment because they both use the 7th generation Intel chips, whereas the 8th generation offer a nice improvement and are shipping on other laptops.
Your mix of development/design/mobile development is important to consider - there are certain tasks which may be harder or impossible on Windows.
As others have mentioned Ubuntu or another Linux distribution can also be well suited to development (but again comes down to requirements).
People can quibble either way on some, but generally in 2018 these rules still apply:
If you do any mobile development (including React Native/Flutter) then macOS makes most sense for access to the iOS simulator.
If you do some design or work closely with designers and require Mac specific tools (like Sketch) then macOS also makes most sense.
If you use a predominantly Microsoft stack (C#/.Net) then Windows is the way to go.
If you primarily develop with a non-Microsoft stack (Python/Java/Ruby/Go/Rust etc. etc.) then Linux is the best option.
WWDC has come and gone without any new hardware and to my knowledge there are no reliable rumours of Surface updates either (aside from a budget model). I suspect both will get updated around September/October but they may come sooner.
The MacBook has well documented keyboard issues and the Surface Book is bizarrely missing Thunderbolt and has some quality issues of its own (like an unreliable power button). It is a frustrating game of waiting as the next round of updates will very likely address these issues and be “the time to buy” for MacBook and Surface Book.
If you need to buy now then I would pick either the Thinkpad X1 Carbon or Thinkpad T480s. The reason I would pick the Thinkpad models over the Dell XPS 13 is because they both have slightly larger screens (14"), more comfortable spill-proof keyboards, higher build standards and include a webcam in the right position.
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This question is similar (it was Surface Laptop not Surface Book) but you may still find the discussion useful for your decision making.
What's development like with a Surface laptop? Worth switching from OSX?
Sten
I answered there in more detail but to summarise:
People can quibble either way on some, but generally in 2018 these rules still apply:
WWDC has come and gone without any new hardware and to my knowledge there are no reliable rumours of Surface updates either (aside from a budget model). I suspect both will get updated around September/October but they may come sooner.
The MacBook has well documented keyboard issues and the Surface Book is bizarrely missing Thunderbolt and has some quality issues of its own (like an unreliable power button). It is a frustrating game of waiting as the next round of updates will very likely address these issues and be “the time to buy” for MacBook and Surface Book.
If you need to buy now then I would pick either the Thinkpad X1 Carbon or Thinkpad T480s. The reason I would pick the Thinkpad models over the Dell XPS 13 is because they both have slightly larger screens (14"), more comfortable spill-proof keyboards, higher build standards and include a webcam in the right position.