I have used both plastic and brass. The brass Sharkbite fittings hold great. I use a plastic Sharkbite fitting in the drain on my hot water heater with a piece of pex out of it so I can avoid the splashing. The pex does not leak when installed in the plastic fitting but it easily pulls out without use of the disassembly tool that is required on the brass fittings. If you are wanting a permanent connection (which can be taken apart with the special tool) I would definitely use the brass fittings even though they are fairly expensive.
