It's been my experience that they are basically interchangeable. Hole size is actually slightly larger for the Happs buttons. The switch only needs to make a connection for the encoder to read the button press. The rest is handled by the encoder. However, the leaf switch holders do take up a little more room under the panel. Not a big deal really.
If you want to give leaf switches a try and not spend so much cash...
I've got a box of used ones I was looking to get rid of cheep.
Here is link to my post with pictures, etc..
http://forum.arcadecontrols.com/index.php?topic=70958.0I'm selling complete sets (button, switch, holder & nut) for $2 + postage.
Would be willing to make you a 'package deal' if you wanted a lot of them.