I have only used OS and Saito. I have a saito 100 and a OS 120. I must say I love how easy the OS was to tune, and even out of tune it still runs smooth. The saito will run out of tune but is will knock really bad and shake you plane. Both my saito and os run smooth as glass but I would do a OS as a first 4 stroker. I have never seen a magnum in person but they look just like the OS and they probably run just like the OS and they are cheeper than the OS ( about $100 difference on all sizes). So if money is a big factor then I would go with a Magnum.

I have ran 2 strokes and 4 strokes. To me if you want speed, go with a 2 stroke and if you want scale or something that will give you a lot of pull power go with a 4 stroke. If noise is a big issues in your area, I would go with a 4 stroke.

Also saito are very light so, if you need noise weight I would not go with a saito or you could go with a saito but you will have to go with a bigger engine.