I did my CPL on a single. I now teach CPL at a school which offers both options to students, although most students choose to do the CPL on the multi.
I think it's probably true that it's slightly more difficult to do the CPL on a multi, but I really don't think it's a huge increase in difficulty over doing it in a single.
Personally, I wouldn't let this influence your choice of school. There are far more important factors. If you find a school which you like, then the aircraft they use really shouldn't affect your decision to train at that school.
One thing I would be slightly wary of, though, is doing the CPL partly on a single, then finishing it on the multi. This is an option which my school offers, but one which not many students take up. Those who do find that, when they go for their CPL test, they have relatively few multi hours (compared to someone who's done the entire course on a multi), and this is a big increase in difficulty IMHO.
FF
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