This is the combo that I will have here shortly. I'm looking to get a new MacBook Air 2018 and recently purchased the iPad 12.9 Pro.
I've had my MacBook 12" 2015 for a few years now and have enjoyed it - but need just a bit more, so the Air should suffice well
I am also having 12.9 iPP and I think the MBA 2018 is not a good addition to it. The iPP beats the Air in mobility and performance (by a huge margin). I would suggest to stay with 12“ MB, the MBA is no real improvement.
If you need more performance or a better display buy a MacBook Pro or an iMac. If you just want to compensate the limitations of the iPP you might buy a cheap Surface Go, as I did. That device has the right price/performance ratio and is even lighter than the iPP 12.9. You also have trackpad/mouse support, all kind of pro Apps (not the mobile versions), a good backlit keyboard and the iPP can be a second display for it with Duet....