You should split this do Dutch/German forum but here are my 2 cents.
it is Amsterdam - by all means.Every minute of it.Compared to Amsterdam,Frankfurt is ultra-flat.It is just huge business center raised on carpet bombing ruins. (If for some reason Amsterdam becomes problem than go to Mainz & Wiesbaden.They are 15min frequently connected train distance from Frankfurt Airport and those are places worth a look.)
I've been to both, and I would definitely pick Amsterdam.
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