Mini DisplayPort to DVI/VGA Adaptor for projectors
I have the original one and it works just great. From my experience, ~99% of the projectors have a VGA port, some DVI. To be on the safe side, I would rather pick VGA than DVI if I'd had to choose one. I cannot say much about the non-official VGA adaptor, but I am using the original VGA adaptor for years now, and I am completely satisfied. But I also have a non-Apple Displayport to HDMI that is very reliable (Rocketfish).

In theory, DVI should have better quality than VGA if the beamer provides the appropriate output, though, since the former transmits the signal digitally, and VGA is analog. Principally, DVI is HDMI without sound.

I can remember some colleagues complaining that their generic once were sometimes not recognized. But this could also have other reasons and happen with the original one.