I haven't played with the Heym, but have compared a modern Merkel against a modern Chapuisse - both very nice although the Merkel felt more solid (in fairness, the Merkel was in 470 and the Chapuisse in 450/400, that may have accounted for the difference on weight alone).

Aside from aesthetics, which are a personal preference, there isn't much to split them on quality wise.

The Chapuisse had a big gap between the triggers which made the right barrel a bit of a stretch - the Merkel has an articulated trigger which is nice and close - again, a preference issue as opposed to any real difference.

I'd be interested to hear what the supposed issues are with the Merkels.