I'd say, if "your" party nominates a dog of a candidate, then you do "your" party more favors by communicating your dissatisfaction by abstaining.
But this whole notion of "support your party over your convictions" frankly is the worst facet of politics, and has echoes of Soviet politics to it. The fact is, I've always voted for a particular party (guess which one!
), but I'm registered Independent, and if an independent or third party candidate (or, I suppose, a candidate of the "other" party) came along I could get enthusiastically behind, they'd have my vote.