The difference between pex and pb is basically how it's made and it's properties, have a little read of this
http://www.pexmall.com/difference-be...ex-and-pb-pipe
I'd go for the better quality pipe like the JG stuff, a Tim says don't forget the inserts and make sure they're the right inserts for the pipe and fitting. E.g john guest has 2 types of insert the superseal and standard. The superseal is only to be used in JG push fittings but if you're connecting a compression fitting to the pipe you need standard insert and copper olive in the fitting as the brass ones are too hard. Also with the JG ones make sure you tighten them once the pipe is fully in otherwise they will leak at some point.