My brother and I have several OS four strokes (70 & 91 sizes, not pumped) and they seem totally reliable and are easy to tune. They seem to have a pretty broad adjustment range on the needles.

We also have several Magnum four strokes (61, 70 & 91 sizes). They run very well and are very reliable after break in. Break in is longer than with OS but not bad. The older models (without the oil return to the intake pipe) take a lot longer than the new models. Our experience has been that the Magnums have a pretty narrow adjustment range on the needles. They also don't seem to like running at all rich at idle.

As far as power, with a 14x6 prop and the same fuel, some of the engines of each brand are stronger than the rest.

I like the long plastic throttle arm on the OS better than the metal arm on the Magnum. It makes it easier to hook up throttle linkage. Overall I prefer OS but buy Magnum.