Which would you prefer? Olympus 75 1.8 vs canon 135L 2.0

I would buy the suitable lens for whichever system you want to use more...

The 135L on a 5dIII will be the better quality overall - the 135L is one of the best lenses out there, which coupled to a high end full frame camera like the 5dIII should give you some stunning results. The fact is though that this means a larger lens on a significantly larger body - if you are happy carrying the weight, then go for it.

The fact is though that you already sounds fairly happy with your OM-D, so presumably the difference in quality is worth it for the lesser size and weight, and features like IBIS and so on. If you have been finding yourself grabbing this setup more often then going for the 75mm (which while not having quite the reputation of the 135L is a superb lens) probably makes a bit more sense.

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