I would go so far to say there isn't actually a dichotomy here -- you should be swapping between launching something with a hypothesis (in lean mode) then gathering feedback and considering alternatives as you are proven correct/incorrect (hammock mode). I think Galls Law  is also relevant here:
"A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system."
If all you do is think and think, then you open yourself to mis-timing a solution, feature and scope creep, and risking "unknown unknowns". If all you do is launch and incrementally iterate you'll be stuck solving very narrow problems.