Or in other words 200 hours work versus 600.
Unfortunately the JAA CPL exams do not qualify you do do the IR, at any time. The IR exams are now a seperate set of exams but are included in the ATPL exams; whatever you take, they are always valid for the FI course, but only have a 36 month validity to complete both CPL and IR. If you don't need the IR or intend to fly large aeroplanes there is no advantage taking the ATPL writtens.