I opened a thread on this very issue but I didn't receive any replies so I telephoned Briggs and Riley today. From the information I received there you won't lose space with the International because it can expand to bigger than the Domestic and therefore has a higher cubic capacity.

I have been umming and ahing whether to go for the Tumi Alpha 2 Continental Expandable or the U121CXW. In terms of dimensions I note the Tumi is 56x40.5x23cm vs 53.x39x22cm for the U121CXW. I still haven't made my mind up.