Weather in Donegal - as in the rest of Ireland - is unpredictable but may feature rain and lowish temperatures in November, and Donegal can be bit miserable if you are stuck indoors due to bad weather. However, assuming that you get a least some good weather and that you also have a car to get around, you should be able to get out and about for some of the time.
Location wise, Rathmullan and Buncrana are both small towns - though Rathmullan is probably no more than a village, whereas Letterkenny is more of a busy country town. Letterkenny has more facilities of a commercial nature including a shopping centre and various commercial outlets. Buncrana and Rathmullan benefit from seaside locations and are prettier towns.
Letterkenny has a range of hotels and B&Bs in and near the town. Rathmullan has a more limited range - Rathmullan House is a good hotel wthin easy reach of the village centre and it has the benefit (assuming some good weather for walking etc) of being on a beach. Frewin House B&B near Ramelton (between Rathmullan and Letterkenny) also gets good reviews. Buncrana has a number of hotels and B&Bs - probably a better range than Rathmullan.