Good idea to see which FL suits best, and MPB are pretty good, have bought used from them a number of times. I see the prices on the 25 haven't fallen much, I sold mine for £110 like new a while back. But, for £120 it's still a steal I think, very easy lens to get on with. I've not used the 20mm but I do know it's noisier and slower to focus, both have excellent IQPersonally, I'd buy second hand, and if you end up not using it much you can sell it on without losing much.
Try your 12-32 at both 20mm and 25mm to see which focal length suits you best.
How does the olympus 25 compare with the panasonic 25?The only 20mm I can think of is the Panasonic 20mm f1.7.
If that's the one... it's compact and has good image quality but the focus is relatively slow when compared to other lenses. I replaced mine with the Olympus 25mm f1.8.
You could also think about the Olympus 17mm f1.8 as mentioned by AMcUK. It's an excellent metal bodied lens with a good manual focus option with end stops and markings. I think all MFT lenses should be built like it
Good luck choosing.