My two cents: I recently tried 2017 MBP nTB, 2018 MBA and 2018 MBP. If we take performance with what I'd call 'normal use' - a mix of web browsing, watching video and conf calls - then MBA feels somehow light - it works fine, but you feel the boundaries as soon as you start having video conf calls in either Skype or Hangout, or even FaceTime (the latter is optimised for MacOS and still takes about 60-70% of MBA CPU). MBA withstand video conf load for several minutes and then it starts spinning the fan at maximum RPM - which is way too loud for just a regular Skype call! It feels that this machine is effectively a type writing machine with an occasional extended use. Yes, I know about 4K editing - but where? in heavily optimised Final Cut Pro only. Is all software you want/need to use made by Apple? In my case - unfortunately not, so I felt the boundaries quite often.
As for the smoothness - well, MBA GPU - Intel UHD 617 is weakest out of three, and although Retina resolution on Macbooks is there for more than 5 years now, it appears quite challenging sometimes for integrated graphics. Even MBP 13 2018 with Iris 655 struggles from time to time (especially while changing desktops). This is why non-Retina MBA works smoother - as its GPU albeit being even weaker, doesn't need to support that high retina resolution.
Sad but MBA 2018 simply doesn't have enough raw power to couple with non-optimised things (MS Office, Skype, Google Apps etc.). Ultimately, I think it all depends on the intended purpose. I would not take MBA if you know that you may need to stretch it (using non-optimised software, especially Chrome-based, like Hangouts) - it's just not designed for this (poor cooling system in the first place, CPU throttles quite a lot).
Saying this I found MBA the most pleasant Macbook to operate - it's a joy to hold, use in a various conditions (laying on the sofa, sitting on the train, using in a small meeting room). If only I could allow myself to limit my software list with Safari, iTunes and Pages/Numbers - I would take it in a heartbeat! It's great, but it's Casual not Pro, and it has inferior components (primarily screen and webcam), so taking MBA you need to switch off your rational thinking!
A few words on 2017 MBP nTB - good choice, but a few BUTs for me: a) don't want to spend time in genius bar waiting for keyboard replacement when it fails - which seems to be almost a given with 2nd gen butterfly - don't care that it's a free of charge, our time is the most precious thing! b) subjective, but i like the feeling and the sound of 3rd gen much better - had enough time to be certain c) MBP nTB has compromised cooling system with 1 annoyingly high-pitched fan, so the laptop gets either quite warm (well, pretty much as 2018 MBP but that's whole different story) or noisy.
Hope this helps - spent lots of time with 5 replacements. Good luck with your purchase!
