As much as I love Foundation, bootstrap is my new default because of its heavy use. There are a lot more tools, snippets, templates and error explained for bootstrap. I hope the BS3-4 migration will be a smoother compared to the last one.
In contrast to the poll above i would love to not upgrade, but i know that's not an option. So an upgrade is inevitable. Apparently the upgrades of these frameworks are mostly backward incompatible which is quite a drag if you maintain a lot of applications. But if i have to choose, it would probably be BS4.