I played around with several of the low/medium output HBs and have these thoughts for you:
The Riff Raff is my favorite HB. In that family, I like the Rebel Yell and Nailbomb as well, which to my ear are higher output versions of the RR.
I liked the Mule at first, but upon spending more time with it, found it boring compared to the RR. I think the difference is the A5 magnets on the RR giving it a punchier response. The Mule is for sure more true to the vintage PAF tone, but I think it's more balanced in the treble (compared to a Seth Lover I had) so it's not overly bright. IMHO, the Mule sounds less dynamic than the RR, though. If you like a super well balanced PAF sound, the Mule is for you. If you like more of a hot rodded PAF, get the RR. There are certainly cases where the Mule will do better for you depending on what you play but for me the RR is super cool.
Regarding the Youtube video showing the RR/Mule combo and saying it's the best combo of pickups on the planet, I have to disagree a little bit. I find the Mule in the neck doesn't work for me. It sounds thick and muddy. I did a lot better with a RR in the neck, which sounds slightly glassy (almost like a thicker, souped up Strat neck pickup) or a HSP90 in the neck (Nantucket), paired with the RR bridge.