Given the faintness of the Maling-type plaque's imprint, it seems fair to assume it was produced later. So based on this small sample, a plausible explanation would be that Sheriff Hill made plates and small plaques from 1830–1845. After that, their transfer plates found their way to Maling.
Below I've compared a late Sheriff Hill-attributed plaque with the Maling-type plaque. This time with no adjustment of contrast. If you click on the centre detail of the second plaque, you'll see that despite its faintness, it appears to have been re-engraved. The lines on Wesley's wig have been picked out in rows of small dots, that don't appear on the earlier imprints.