I think one of the main attractions of the AGM M4 is that you could potentially build something as reliable (or more) for the same price or less than the landed price in Canada, and it comes with a metal body. Another advantage is that the WA system has a lot of upgrades going for it, as well as many different companies making mags/accessories, whereas it seems to me that it's almost exclusively WE offering replacement parts or upgrades.
That being said, I'm not exactly "in the know" when it comes to GBBRs, so I don't know who's coming out with what in terms of upgrades, etc. I will however say that if you get a WE, then you won't need to buy new mags when (and if) the cansoft WE SCAR comes out
