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You don't want the kludge of an adapter. The difference between 1.4 and 1.7 is almost nothing. Plus the 20mm will give you a slightly wider field of view which will be more handy.
Half a stop is hardly *nothing*
If low light is you biggest concern go with the Sigma 30mm f/1.4.
If auto focus is a concern go with the 20mm f/1.7 Pancake.