Your understanding of ADAT is correct and I find it useful as a widely supported way to
connect gear together without any grounding issues as it provides electrical isolation.
That looks like a good price for the Focusrite - especially if it includes the
ADAT card but be aware that some people who know the products say that the LE version uses
a better preamp design. I would also be careful with the Alesis interface - it was one of
the first DICE II based units and reports I've seen show that it seems fussier about what
it will work with than later models from other manufacturers. I'd have a good look around
before choosing an interface.
JRP Music - Audio Mastering and Restoration.