Before I came to cubecraft, I've spent about 3-4 years on a server with skyblock and towny survival.
I can tell you that both of them were equally as popular.
Skyblock having challenges and trying to get the best kind of island while on survival, you try to get the best town and residents to expand the area.
The issue about towny survival or survival itself, because cubecraft mainly consist of PVP, we're going to have a ton of griefers who will mistake survival to be the same as factions and just raid and raid and oh, did I mention raid?
So that is going to be one of the main issues for survival.
"But it has a towny protection to prevent grief"
Not all players will be able to afford a town or stay in a town, so some of them would just build in the wilderness which is not raid-proof.
From my knowledge, towny protection is not 100% safe.
I support this idea but we would have to find a way to solve those issues as it would require tons of rollback.