I think that I summed it up previously but let me expand.
Yes the course content may be similar if in fact the same.
But the standard of flying, cognitive process, SA, capacity, captaincy, decision making, pre-flight planning, forward planning, passenger comfort awareness, safety awareness etc and all around piloting will need to be higher.
That's not to say I haven't met a number of PPL holders that could fly circles around CPL holders but alas the course is the course.
Have a look at the tolerances allowed for the PPL and CPL skill tests and you'll start to get an idea of what I'm talking about.
http://www.caa.co.uk/docs/33/srg_lts...8_Sept2012.pdf
http://www.caa.co.uk/docs/33/srg_lts..._V7_Sept12.pdf