The 2018 Air is not slower than the 2017 nTB Pro by a significant margin. I would say it will probably have similar performance in normal day to day tasks. There's not much difference in their geekbench scores. And the RAMs and SSDs are similar in performance. The graphics performance is better in the Pro.
If people need that minor improvement in performance, sure get the Pro. But its not something most people NEED or notice in their day to day life, its just people here in MR who obsess over things like Geekbench scores. I think if you need the performance you're better off getting the TB Pro, while the Air is the best bet if you are a casual user who wants a everyday laptop.
The Air is $100 cheaper, and has way more practical improvements over the Pro, which people will notice and love, over it. Like the fact its lighter, has a tapered design, has touch ID, and extra 2hrs battery life. Not to mention it has the 3rd gen keyboard. All real practical considerations.