I've never tried an ED50, but I did a fair bit of internet research on these two scopes before buying my MM4 60mm with SDLv2 zoom earlier this year. One of the things that clinched it for me is that it is very difficult to try a Nikon scope before buying in the UK, as many optics retailers don't sell them. Also, Opticron has a much wider range of eyepieces, as Nikon have discontinued the fixed eyepieces (which are generally considered the best fit for an ED50) and only sell the zoom which, although very sharp, is said to have a narrow FOV. Both MM4 scopes are noticeably heavier than the ED50, but this is partly because they are more solidly built. The Opticrons also have a rotatable tripod collar, which has proved useful to me on numerous occasions. They also have better eye relief, but as I do not wear spectacles, this is not something I took into consideration. Overall the consensus seems to be that good copies of the ED50 and MM4 50 are fairly close optically, but that the MM4 60 has a clear advantage due to its larger objective lens size (but is also larger and heavier). I think that, if I wanted a very lightweight scope for hiking, I would buy an ED50 with a used fixed eyepiece, but for more general use, I think my MM4 60 hits the sweet spot. The choice between ED50 and MM4 50 is a more difficult one, and comes down to smaller size/weight v. better eye relief, tripod collar & eyepiece availability. Oh, and the Opticrons also have a 30 year warranty, and I've heard good things about their customer service. Not sure how Nikon compares in this respect.