Unitek Y-247A
vs
uGo UKD-1086
vs
Creative SB Play 3
each card was tested by connecting its input to its output with a janky cable and running
play -n synth 6 sin 20-20000
while recording with audacity.
This is a somewhat flawed, since I'm including each card's input path in the comparison, but at least both input and output run on the same clock, and only one USB card is in use at a time.