In my opinion, the 720p videos are blurrier than the 1080p when scaled to full screen on a bigger than 720p monitor. And recently I bought a 2560x1440 screen for which I prefer the videos in that resolution (although those are still quite rare). Also, I enjoy 60 fps videos more than the 30 fps variants and I regularly watch a few channels, that have such content. But these are personal tastes and everyone should find their own sweet spot.
On a more on topic note, I found an alternative that actually streams and plays mp4 dash formats really well: mpv with youtube-dl using the firefox extension Watch with MPV. This has been able to handle anything I have thrown at it until now (although I haven't done extensive testing). The down side is, it's a bit more hassle to set up and you have to configure and learn the keyboard shortcuts for a new media player, that is very minimalistic in design, which I incidentally like!