There are at least 3 different Olympus 14-42mm lenses!
Olympus M.Zuiko Digital ED 14-42mm f/3.5-5.6 EZ
This one is very small - it has a motorised zoom and extends when you turn on the camera.
It was bundled with some of the PEN series cameras and the E-M10 range
You can see my use of it in my out of date Flickr album here
A great pancake zoom lens for micro 4/3rds
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Olympus M.Zuiko Digital ED 14-42mm f/3.5-5.6
and the upgraded
Olympus M.Zuiko ED 14-42mm f/3.5-5.6 II R
These are manual zoom lenses. They manually collapse so they're smaller to carry.
You twist the lens to open it up but need to press the lock button to make it collapse again.
It's a neat robust way of making the lens smaller without slowing you down much.
I have the II R which I got with my first E-PL5 and the EZ which came with my E-M10 Mki.
Optically I can't see any difference at all, doesn't mean there isn't one but nothing I ever noticed.
They're both very light and very compact. I used to favour the EZ over the II R because it's smaller.
For use, the manual zoom on the II R is more comfortable to use - you can go quick or adjust more finely than the motorised zoom on the EZ.
I have read reports of the EZ showing problems with decentering - where part of the frame is sharper than other parts because the elements aren't perfectly aligned.
I have also read reports of the motor failing.
My own lens showed none of those issues but I'd guess a small motorised zoom is less resilient than a larger manual one as there are more moving parts in a smaller space.
On the E-M5 I'm not sure you're seeing enough benefit from the shrunken size to make it worth a
second hand purchase but you might choose to do so.
I've never used the Panasonic zooms so I won't comment.
The 12-50mm Olympus was bundled with the E-M5 so it's a very good fit. Personally I'd like the 12mm wider end over 14mm.and the 50mm is more useful than 42mm.
With M43 those small numbers add up thanks to the 2x crop factor.
Finally - I saw a used 12-40mm PRO Olympus lens when I was feeling flush. That lens
transformed my photography.
It's bigger and heavier than the other two lenses and much more expensive but it has barely left my camera since to the expense of my f1.8 and f1.7 primes.
If I had to choose one M43 lens then that would be it.
If you can afford to buy a good one used you should see most of that back if you chose to sell it on again later.
Again here's some of my stuff from my derelict Flickr
M.12-40mm F2.8 PRO M.Zuiko Digital ED 12–40mm F2.8 PRO
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