I was in a similar situation - except that I wanted to do the IR with a CPL some time later... so could do either the separate CPL and IR courses or ATPL in one go.
I went for ATPL as there is no reasonable upgrade path from CPL and IR theory to ATPL - its all 14 exams plus all the ground school again! You never know... and I am reasonably certain that I will complete both IR and CPL in the required 3 years.
If you do ATPL theory, keep in mind that it lapses 3 years after CPL if you do not do the IR. If you get the IR, it "lives" as long as the IR is valid, and for 7 years thereafter.
If you are sure you don't want the IR later, I would not bother with the ATPL theory - saving about half of the work required, I think. GTS does separate CPL and ATPL theory with the same ground instructors and abridged versions of the ATPL materials, so course quality should not be an issue. Can't comment on other schools - as usual, you only need to attend one if it is any good...
C.