8 analog inputs
+8 ADAT optical ins
+2 S/PIDF
--------------------
18ins
I think the AltSource inputs that you're refering to are RCA and just take over the 7/8 ins....so if you have an RCA device to hook up you can do so without having to buy or make an RCA to 1/4" cable. Same with the ALT Main out....except they go to speakers with RCA.
and yeah, if you're constantly needing to record 24 channels at once, it might be wise to get an HD unit. but if you dont' want to spend the money, think about if you REALLY need to record 24 at once or if you can get away with doing like 12 at once, 4 later, and so on.