My personal opinion is that copper is better, mostly because I have concerns regarding the potential longevity of the joints. In simple terms, since the o-rings within pushfit connectors are presumably made from some kind of rubber or silicon compound, and we all know about leaks from ageing silicon around things like baths, and how rubber degrades over time, then what chance will something that has to undergo vibrations from water flow, changing pressures, plus variations of temperature (potentially from freezing up to maybe 70+ degrees C)have?
Couple that with plastic itself always becoming increasingly brittle with age.
I think that the life expectancy of plastic is highly questionable.
But others will no doubt differ and cite anecdotal evidence in support of this.