You can choose if you go for Interrail (home country limitations, only two travel days possible in your home country) or for Eurail (no limitations), same price though.
There’s also the one country pass Germany
if you like to plan well in advance, you can better buy cheap advance tickets. (E.g. Göteborg-Hannover can be done for +/- 55 EUR per person, which is absolutely not bad for a 10 h train journey).
So plan a bit where you would like to go, and compare the cost of a pass with ordinary tickets.
If you don’t really know what you want to visit, and value a lot of flexibility then Interrail/Eurail is your go. If you’d like to travel close to Christmas, Interrail/Eurail is probably going to be the better deal, since a lot of people will be travelling for the holidays, thus raising prices.
Note that there are no timetables available yet after December 10.