CPL is harder in my book.
You spend a large portion of the time dead reckoning with no positive idea of where you are, where on the IR you pretty much always have an instrument position fix.
With the CPL every radio call you make is a very long winded introduction of yourself and your flight plan, on the IR you are expected every step of the way so make little more than a position report.
With the CPL you are assessed all the time not only on your accuracy of flying, but also looking out for other aircraft and not flying into crap weather. On the IR you can pretty much just fly as if on rails looking only at the six instruments in front of you.
In the CPL the emergency will mean having to choose a field and prepare for a crash, IR you just keep on climbing with one foot hard on the floor.
On the whole I found the IR a lot easier, it made more sense and there was a lot less pressure on the day, particularly with things like appropriate weather.