The simplest analogy would be RecalBox is like Apple, RetroPie is like Android
Recalbox is a nice project where their emphasis is on user friendliness. It’s built with a buildroot system so it is streamlined for performance but very much hardcoded in, and obviously is much more limited in what you can do, has less systems, less settings to play with etc – but its a good plug and play option for those that want a simple experience. They’ve also done a lot more work on their fork of emulationstation so that it includes music in the background, favourites, etc.
RetroPie as you realise has a lot more options, is much more modular and can be installed on top of more than just the raspberry pi (x86, Odroid (sorta), and likely other devices in the future) Because it’s installed on top of an operating system (like Raspbian on the Raspberry Pi) It’s a lot more flexible for the kind of stuff you might want to kludge onto it (we just added 7 ish game engines in the last week) so power users can pretty much do whatever they want if they know what they are doing.
I obviously have a bias as I’ve worked on retropie for over a year now (and not to toot my own horn but the documentation for RetroPie is fantastic)
RetroPie also has a more active community. You can look at the download counts for both projects to get an idea of their relative popularity:
I wouldn’t say any one is better than the other as they both have unique things to offer, rather it just depends on what your priorities are and what you prefer.
And also just to add the majority of the software retropie and recalbox use are essentially the same (RetroArch and EmulationStation), the main difference lies in the environment the software is built on and the settings available.