The only difference between the HD24 and the HD24XR is the converters. The stock model will only give you 44.1k or 48k on the analog inputs, but the XR will give you up to 96k analog, limited to 12 tracks.
I think they will both do 96k digital.
Most people (including Alesis) will tell you that even at 44.1 or 48, the XR converters sound better. I have the XR model and think it sounds fantastic, but haven't compared it to the other model.