I am sorry, there was some miscommunication here: I really meant that both towns/settlements are equally good candidates for your search - as opposed to other places from distant parts of Poland. Both are relatively close to Płock, both used to be part of Russian Province of Płock. I really have to add one thing with respect to Golub-Dobrzyń: Golub used to be in Prussian Partition, while Dobrzyń was in Russian one. Drwęca was a border river at those times. This may have something to do with your grandmother's reference to Pomerania. And historically, dating back, both towns were subjects to frequent border wars between Poland and Teutonic Knights.
I am not involved in any genealogy project, so I have no family search experience and I cannot provide you with any standard feedback regarding availability of records. All I can suggest is based on some common sense. To start with, you may try to dig some information about the perceived size of your grandfather's "Dobrzyń". Dobrzyń and Wisłą - even though historically old - is actually very small: it currently has 2,246 inhabitants. The "Dobrzyń" part of Golub-Dobrzyń, on the other hand, has 10,071 inhabitants. Apparently this "Dobrzyń" is few times bigger than the other one. I can only assume that the similar ratio existed in 19 century.
The next thing to check concerns religion. You seemed to be implying that your great-grandfather became Catholic just of the great devotion to his wife. So what religion was he born into? This might be an important piece of information. For example, Golub-Dorzyń is accessible via JewishGen Locality database, suggesting that there were Jews living in that area. Check for example this: data.jewishgen.org/wconnect/wc.dll?jg~jgsys~community~-502272, or this: jewishgen.org/jri-pl/town/golub-dobrzyn.htm.
I should also add that - as suggested by other posters here - the name "Dobrzeń" could also be considered as one of the candidates. My cursory search shows that there are two such places, but they are located in Silesia and Opole Voivodship - South-West, way away from Płock.
Dobrzyń and Wisłą
My overzealous, self-correcting editors drive me crazy sometimes: this should obviously be "Dobrzyń NAD Wisłą", not "Dobrzyń AND Wisłą". I am sure I typed "NAD".