Rationality is only one aspect of truth. In the logical arena, I suppose many NTs excel. I've been over at INFPgc for about a year and I seem to be more obsessed with debating ideas than most INFPs there. So, my experience is probably far from representative. I'm fairly argumentative for someone who prefers to avoid most conflict. For whatever reason, intellectual conflict inspires me. Then again, INFPs can deeply value anything including rationality.
The problem with people interested in 'truth' are issues of intellectual honesty and humility. Most NFs I've met are fairly honest and humble about what they don't know. If anything, many NFs are by definition less interested in objective truth because Feeling gives a subjective slant to their thinking. And so the weakness of NFs would be the dismissal of objective rationality. Even so, I supsect NFs are better than NTs at thinking about thinking(ie seeing the subjective motivations and underpinnings of supposed 'rational' thought). In this sense, I think its no accident that it was an INFP that worked out a fairly complex system such as the MBTI.
NTs are more interested in the structure of thought.
NFs are more interested in the substance of thought.
I idealize truth in all of its forms. I appreciate the clarity of thought that many NTs bring to the table, but many NTs are too self-assured in their thinking sometimes to the point of self-righteousness(not to say other types can't also fall prey to this). This annoys me, but not only because it shows a lack of social graces. Too much self-assuredness in rational thought can lead to rationalization and a blindness to other aspects of thought. I've had discussions with a number of NTs where they seemed utterly clueless about the emotional undertow to their own thinking. There is a thin line between rationality being feuled by unconscious motivations and unconscious motivations being rationalized.
Sometimes, the data and the logic don't speak for themselves. NFs are the masters of meaning which is what gives to ideas a larger and deeper context. NTs too often argue for the sake of arguing, and especially with INTPs this can seem to go nowhere. I've never been fond of intellectual pissing contests.
Both NFs and NTs are less connected to the realities of Sensation data. The abstractions of rational thought aren't necessarily any more realistic than the abstracting tendencies of NFs. All types see reality partially. NFs only have some advantage in the sense that they're hyper-aware of this paritality(subjective bias).
NTs are better at finding the distinctions between ideas.
NFs are better at finding the relationships between ideas.
NTs are better at analyzing and systematizing mental constructs.
NFs are better at seeing the context inherent to multiple perspectives.