This one is a single flatpak that is self contained, the other one is a script that downloads the emulators one by one, in both cases you can pick and choose which ones you want to download from a list and they'll do the rest if you provide the ROMs
Yeah at this point they're basically the same thing. The important difference you have to know is that Emudeck will install emulators independently, and show them as their own games in your Steam library, whereas Retrodeck will install ES-de and only show one executable in your Steam library, from which you will launch your emulated games. It's a matter of preference.