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The 150mm F/8 is a heavier scope than the other two you mention (~10kgs for the 150mm versus ~6kgs for the 120ED and ~7.5kg for the Vixen). The 150mm is also much more nose heavy than the other two scopes making it a pain to mount. If you want to look at objects near the zenith with a 150mm F/8 scope you'll need a tall tripod/pier if you want to avoid getting close to the ground (4ft long OTA balanced about 1/3rd back from the objective).
I had a Synta 150mm F/8 and its 120mm F/8.3 sibling and much preferred the smaller scope for its ease of use so I sold the 150mm OTA. Subsequently the 120mm Synta made way for a Vixen 4" F/9 ED scope. I like the Vixen fit an finish (and focuser) much more than the Synta but between a 120ED and a NA140SS I'd probably take the 120ED, sometimes I'd want to bump the magnification up and today the false colour bugs me. Happy observing with whatever you pick!
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