Hello LorenzSingapore
A seat reservation is not needed on a Tuesday in mid December, but would be a great investment on weekends, during summer time.
I strongly suggest you to wait with the purchase of the tickets until you are on-site. No local would buy a mountain ticket a month in advance while not knowing the weather forecast. A Jungfraujoch visit is a stunning experience with good weather, otherwise you will pay a fortune to finally end up in the clouds and fog.
Kind regards,
Roland