It sounds to me like you need both. If you need to evaluate cards that only go in a desktop, then you need a desktop. If you need something occasionally portable, then you need a Macbook or a Macbook Pro (or possibly even a Mini). I would suggest modest but not underpowered versions of each.
And yes, we run an interracial network here at the office, although I don't know the details as to how, but it can be done.
Also, I don't know how powerful the Air is, but it seems ideally suited to music listening applications if it's powerful enough.