I have no preference for any particular lens mount and I take them on at a case-by-case basis. The Zuiko has great reviews as does the Sigma Super Wide I mentioned. The Zuiko will obviously cost more as it is a brand name but this will help in resale value as well. The Sigma and Cosina brands are also worth looking at as it is a well-known fact that those two, as well as Tamron and Tokina, make lenses for the major brands. So, sometimes, you will see that a 24/2.8 will have the same optical configuration between Cosina and Canon, that's because they are in fact the same lens.
Something to consider is the adapters required for each lens. If you have lenses in FD, MD, OM, and F-mounts then you will need an adapter for each. Or, you can choose to go with one and make it a little easier. I myself, choose a lens, then buy the correct adapter, and keep it on the lens so that way I don't have to swap mount around all the time and I just need one type of end-cap.
Any ways, I was recently shopping for a 24mm/2.8 and I was deciding between a Zuiko and a Sigma. I went with a Sigma as they can be readily had for under $80 with shipping. Sometimes, if you are lucky, closer to $60. The Zuiko's at the time I was looking were readily over $120 or so and I did not even know if I wanted this focal length so I went with the Sigma. Plus, this focal length is covered by my SEL18200 so it was not a "priority" focal length.
One cool thing about the after-market brands is that a Sigma Super Wide II 24/2.8 is the same lens in any mount. The only difference is the mount so you can search and find a broader array of auctions to look at.
If you are on a budget, Sigma Super Wide II, not on a budget, Zuiko.
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