Ah, you could be starting a topic here that may rage on for some time as experts all argue their preferences.
In principle, there is little to choose between the two as they have both been around for a long time and perform equally as well. The only difference is that Pex has a smaller internal bore, so you can't mix inserts.
Personally, I have both on the van and use them in equal amounts. Generally, I choose Polyplumb (PB) more for heating, simply because the fittings are smaller than Speedfit (B-pex) and the guy I work with never uses b-pex.
I prefer Speedfit for potable water because the fittings have double seals which probably makes them more reliable for the higher pressure and also the pipe is more likely to be visible, behind pedestals / in cupboards etc.
Having said all that, I use copper wherever I can.
HERE WE GO AGAIN GUYS (BUT I LOVE IT)
We had a debate on the pros and cons of the copper plastic debate and copper won but only just i think the phones votes were rigged?
my understaing of john guest piping is that the "barrier" piping is best suited for central heating systems and the normal piping for potable supplys
also if you use john guest piping and use a compression joint to join two lenghts of piping together you use the plastic insert with no o ring on it?