I love my SMH-10. The SMH-20 looks like it has everything I love about the 10 and adds more, plus improving on the existing.
I have come to realize that the one thing that would TRULY make the SMH-20 better, at least for me, would be if it had one (or two!) buttons that would function as a garage door opener.
They make some really tiny garage door openers these days. The guts of one, without having to include a battery (since the SMH already has a battery) would be even more tiny. Making your next model that much bigger and including a garage door remote in it would be sufficient reason for me to upgrade from my SMH-10 units.