MacBook Pro 2015 13" vs MacBook Air 2019 for Final Cut Pro X
Generally speaking, a MacBook Air is not the best system for heavy users of CPU-bound applications like Final Cut Pro X and Logic Pro X. The primary reason for this is that the thinner chassis has less efficient cooling than a larger machine like a MacBook Pro, resulting in the CPU being more likely to throttle down to lower speeds more quickly when it gets hot.

If you can afford it, a better upgrade for your use case may be a 13" MacBook Pro but be aware that a refreshed model is rumoured to be being released this year, with an upgraded keyboard (the same keyboard as in the 2019 MacBook Pro 16" and 2020 MacBook Air).