John,
The SL and Lion Mach2 Music Servers have a lot in common sound-wise. Both have a clear clean sound, with tight bass, rich mids, and open highs. There is a nice solid soundstage with good depth. Music comes alive with a good separation of instruments and lots of detail, without being harsh. Overall, they sound similar. I am happy listening to either in my own system and in our test systems.
There are some
very subtle differences that we noticed in our testing. The Lion Mach2 seems to have a more precise timing. Cymbals sound slightly more crisp, and separation between instruments may be a little better. The SL Mach2 has a slightly richer sound. Again, these differences are very subtle.
Overall, If being on the lastest OSX is important to you, get the Lion Mach2. We offer it in the $1449 2.3 GHZ model, and the $1649 2.5GHZ model.
I personally think the SL Mach2 sounds better overall. SL also will utilize Integer Mode which is important to some people. Lion doesn't have Integer Mode. Snow Leopard is the old OSX so some people won't like that, and we only offer this in the $1649 2.5 Ghz model.
So, like all things there are trade-offs. The good news is that both options sound fantastic and you can't lose with either choice.

Darrell
www.mach2music.com