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I don't see a huge difference between them. If money's tight, or you want the smallest package, go for the Panny 20mm, though some had said is AF is slow. If you want the best optical quality go for the Panny-Leica (would be my vote, though I have none of these lenses).
Whatever you chose, you will probably end up using it a lot. I know I did with my Voightlander 25mm (no complaints at all. Excellent optics - never been disappointed with any shot that's in focus, and probably the best of the several m4/3 primes I own, to include the Oly 60mm. Excellent Bokeh, wonderful controls. It works well with the OM-D's pseudo-"focus peaking" feature for street photography).