I don't know DTS from Adam (or Eve as the case may be), but the fact that he was born in Wuertzburg seems to me a better reason to visit that city than Marx's birthplace makes me want to visit Trier again. I've also heard that Wuertzburg is very interesting in any case.

My personal opinion is that you can't go wrong with either option. I find the Moseltal more interesting than Trier itself. If you choose Heidelberg the Neckertal is great too. Whoever mentioned that there are some great restaurants near Heidelberg was spot on. I don't know how you locate a good one, but the best Spargel I've ever eaten was in greater Heidelberg.