Generally motorways tend to be less busy on Sundays (no trucks) than during the week, but many "Bundesländer" (states/provinces) will be starting their summer school holidays at the end of June, which regularly causes terrible traffic jams especially in the FRA region, as many families would be heading south. So I would also definitely advise you to take a train.

Concerning the museum at Sinsheim: I visited it a couple of years ago and I can also highly recommend it to you. It´s really worth a visit!