The big difference is price/converter. The LynxOne just has 2 ins & outs, and the LynxTwo has been discussed above. I think the LynxOne runs at like $450 and the LynxTwo at around $1000 or more.
In contrast, you can get a Delta1010 with 8 ins & outs, plus MIDI, word clock, and SPDIF, for something around $500-600.
Another thing to consider is that Lynx drivers have been slow coming around. LynxTwo ASIO 2 is still a beta and I don't think either of them support low-latency WDM.
I believe the LynxOne was intended more as a mastering card, and I do believe that it has better converters than something like the Delta, although I have not heard the Lynx personally. I think the LynxTwo just sort of expands upon that idea, and includes crazy converters that can do 200khz! However, the Delta series might be more *usable*, depending on what you need to do. It doesn't matter how good something sounds if it doesn't do what you want it to do.
Everybody wants "as close to pro" as possible, but you might not even be able to tell the difference. You didn't mention what level you're currently at..... a lot of people will swear that they can't hear the difference between a Soundblaster Live! and the better systems out there. It took me about a year before I finally grasped the concept of what my converters were doing to the sound, at which point I upgraded from a soundblaster to Delta44. It took me almost a freakin year after that to finally start hearing the impact the Delta's converters were having, at which time I upgraded to a Delta1010 (because I got it cheap, actually). I can tell the difference between the 1010 and 44, and I can also hear what the 1010, despite it having "good" converters", does to the sound. I still have to record and playback a sound to hear what it truely sounds like.
Without getting too carried away, you can just plug into something by m-Audio or Echo or MOTU or Aardvark and make recordings that are way better than anything a home enthusist has ever been able to make. If you've never used a good soundcard you'll be just floored by how easy it'll be to get a good enough sound. I would focus more on whether the system will work for you, and less on which one will sound more pro...becasue really, pro sound has to come from you. The $300-500 difference between the Lynx and Delta (for instance) can make some room for decent cables or a nice preamp or microphone or sound treatment or......................
Slackmaster 2000