It depends on your application.I have used the Magpul bottom metal on a couple of build, and it works OK. Depending on the stock, it often has to sit proud of the stock for it to function correctly, and magazines often needs some tweaking to sit in toe correct location for them to feed.
True M5 metal is better in every single way.
making statements like "better in every way" is silly and assumes you know exactly what the OP is looking to do. Also important to know is budget and whether or not any components are already on hand.
I have both, have had many flavors of M5 in the past as well. Some of the M5 needed tweaking as well, as is often the case with ANY component you might buy for a project. Application is key, so it is important to explore and understand the pros and cons of all available options.