Soix
Damn that it a difficult endeavor, but I'll give it a go.
Both preamps are fully balanced 6SN7 based preamps (my preferred tube), are dead nuts quiet, background is black as night. Both have great soundstages tall and wide, both have a 3-D like effect with musicians and vocalists clearly placed on the stage, both are very transparent/detailed with even the smallest details coming through, but never ever sibilant. Both preamps are point to point wired without PCBs, use the very highest quality resistors, caps, transformers, volume controls, and wiring. Both are impeccably designed by extraordinary people.
Aric Audio Motherlode XL - The Motherlode XL is full on fun from get go, has more attack and speed, is a little more (I mean slightly) more "tube like" compared to the Raven. That makes the Motherlode a little more "rich/full" with beautiful full tone. I prefer the Aric Audio over the Raven with solid state amps (CODA S5.5, Denafrips Thallo, Pass Labs, BAT Rex 300); and I like it equally well with most tube amps (except 300B)
Raven - Is slightly more transparent and correct on tonality....neutral and slightly less tube like compared to the Motherlode, smooth and clean. It has a magical air about it, one reviewer called it "ethereal" to which I would agree if anyone can describe that. Like the Aric Audio, the soundstage is brilliant and maybe just a touch deeper; and like the Motherlode XL musicians and vocalists are believably placed on the stage. I liked the Raven with solid state amps, just slightly less than the Motherlode; and likewise the Raven is beautiful with tube amps. I will say that I preferred the Raven with the 2 300B amps I've heard (Sachs/Olson Blackbird monoblocks and Aric Audio 300B), and equally well with other tube amps (EL34, KT88, KT120)
It comes down to personal preference and the gear you are going to run in my mind. If one had never heard the other preamp, you would believe you've heard the best preamp ever made. The differences are minor, and they are both brilliant.
I hope this answers your questions Soix