for most of users, it is the same.
i've not followed recalbox from a long time, but as far i the git tells, the activity on the project is very low. only issue fixing.
technically, i changed a lot of things. the philosophy of batocera is more long term, i'm ready to break things and take a risk to add bugs to make them working better. batocera is more agile, regular version to fix more quickly or have user feedback more quickly.
let's take an example, the one refused by the recalbox team and which is at the start of the fork :
recalbox system is a standard file system. when you upgrade recalbox, you unzip a file over this filesystem. it's slow, but the most dangerous is that it's not robust at all. upgrades after upgrades your system because full of files that are not supposed to be there. on the contratry, the first feature which differed on recalbox, is that the batocera system is just a file equivalent to a firmware. upgrading it is just replacing a file. robust and fast. and while it's on sd card and usb key and compressed (requiring less disk input/ouput) it's faster.
for the rest, batocera is based on more recent sofware, kodi linux kernel, emulators.
some other difference it created over the time. you can find the changelogs. for example, i've added hotkeys for several consoles making it easier to save states, exit, ... amiga of course if there and supported from the joystick on x86 and rpi.
something i can't tell is about bugs. some user prefers recalbox, some other batocera mainly because they all don't get the same bugs. an example : on acer aspire one computer, batocera doesn't support n64 while it does on recalbox. this is because on recent linux kernel, intel give up a hack that handled a higher opengl version on this computer. on the other side, recent linux kernel supports more hardware and gpu.
about the team. i don't know the recalbox team of "today". but today, batocera is really more active than recalbox when i were on it. there are regular contributors, not just me.
see commits : https://github.com/batocera-linux/batocera.linux/commits/master
about retropie. yes, it's really more active and supports really more things. but the software is not what batocera intends to be : user friendly, robust (on sd card and usb key, and more generally on embeded devices). batocera is more a long term vision : a firmware bootable on a lot on device on which you've just to plug and play.