When I say "Amish" I mean "Old Order Amish". Mennonites have both lower fertility and lower retention, so their demographic future is less bright overall.

That said, there is a subgroup of the Mennonites, the Old Order Mennonites, who have fertility and retention much closer to that of the Old Order Amish. The Wenger Mennonites, in particular, are rather similar in those measures. But only around a third of Old Order Mennonites are Wenger, and only around 15% of Mennonites in the US are Old Order.