I have a MOTU 424-based system with a 24 i/o and
a 2408 mk3. I have found the sound quality to be excellent, so I voted for it.
There is a lot of flap and gossip about preferences, Black Lion mods and the like (claiming sonic superiority due to the clock and op amps), but I have not seen even ONE valid blind perceptual test. There are many things that can affect your subjective sense of quality, not the least of which is cognitive dissonance. Cognitive dissonance is a result of the mind's proclivity for consistency. If you invest a lot of money in something, you are ceteris paribus more likely to rate it better than if you invest little in it. Thus, without being able to do side by side comparisons, holding constant such things as loudness, dynamics, and EQ, there is no way to know just why some people feel that the MOTU stuff is sonically inferior (unless you like to defer to authority and wealth, which has never been my tendency).
As far as the sound quality goes, 24 bit resolution is 24 bit resolution. Latency jitter is only an issue during monitoring. Preamps and analog compressors (I like tube compressors, personally) are important, because you need to be able to take advantage of the 24 bit resolution. If you drive too hot, you'll introduce digital clipping, which is really awful. If you don't drive hot enough, you can magically transform your 24 bit-width to 20 or even 16 bit quality. This does matter during mixing, because quantization error is cumulative.
I imagine that these devices are comparable sonically, so it boils down to design and feature issues that impact work flow. This is a matter of taste, I suppose.
I recorded my first two albums with a 20 bit Darla card and was happy with the sound quality. The main MOTU advantages are multiple channels, multiple monitor mixes, VST plugins, and an impressive near zero latency monitoring capability with cuemix software. This streamlines my work flow, but in the final analysis, sound quality differences are not that great.
If anyone has access to scientific literature that compares these units systematically, please forward it to me. Thanks... John