3rik

Both are very good, imo it comes down to what desktop environment you prefer.
Main differences apart from that are software availability (both will have most of the stuff you need, but manjaro being based on arch linux often has more when it comes to random niche things, and in terms of customization and kernels etc. Pop will have certain apps that manjaro dosent though.
Installing KDE on pop OS isnt the best idea, since you will end up with duplicates of apps (Gnome Tasks and KSysGuard, Kate and Gedit etc) which is arguably a bad thing. If you prefer KDE its better to use manjaro imo.
Manjaro may also be better for gaming since many of the custom wine builds and beta wine builds (with stuff like EAC support) are packaged for pacman to install, and made for arch-based distros.
And a nice bonus is that if you one day decide to make the jump and switch to full-on arch linux (not that you necessarily should or have to, but you might decide to) it will be easier if you have already used manjaro.

I experienced the same thing going from z370 8600K to B450 3700X with the exact same SSD. Ryzen boot times are almost doubled. Not really an issue for me as it's still under 20 secs from pressing the power button to the os. But definetily something i noticed at first.