I have a maxed-out 2019 16" MBP, and have always been disappointed with the performance. Unsurprisingly the huge issue is the thermals - the throttling means that I never get peak performance from the CPU, and in fact when connected to 2x LG UltraFine 5ks during summer when the temps rise it often throttles to such a degree that the CPUs limit to 1GHz and the laptop effectively crashes. It's also a noisy beast, with the fans running high often and loudly.
I've just purchased a 256GB/8GB MBA and have been using for just less than a week. The silent operation of the MBA is such a wonderful novelty, and the fact that my body temperate doesn't start rising from having a heat brick on my lap is also very welcome - the MBP 16" really is not meant for lap use, whereas the MBA lends itself perfectly for this.
As for desktop use, whilst it only supports 1 of the UltraFines, the performance is outstanding, and largely matches or surpasses the MBP. What I have noticed however is that when connected to the monitor, it can lag in some cases, and in fact I found I had to switch from Safari to Chrome, which seems to handle multiple tabs more efficiently. I'm guessing that the driving of the 5k display puts a lot more pressure on the memory as this is shared between graphics and system. I'm not convinced that the extra GFX core would make much difference, and as the swap storage seems to be capped at 4GB, the extra storage space also wouldn't be needed unless I start to use up the 256GB, which is unlikely. So in my case I believe that the extra 8GB would be a worthwhile upgrade.