The utilitarian case is interesting because both Mill and Bentham seemed to espouse a multidimensional utility vector rather than a uni-dimensional metric. There is an interesting paper I've been considering summarizing that takes a look at this position in the context of neuroeconomics and the neuroscience of desire.

Of interest from the paper: They argue that "pleasure" (liking), though it comes from diverse sources, is evaluated/consolidated at the neurological level as a single sort of thing (allowing a uni-dimensional representation as is common in economics), but show that when it comes to anticipation of pleasure (and the heuristics for evaluating the future rewards an action might yield) we can be quite inaccurate. So effectively, the Bentham/Mill model of utilitarianism is predicated on the perfect or near perfect coherence of "wanting" and "liking" (which does not really exist) and incorrectly prescribes a multidimensional utility measure because of this.

However, Bentham does appear to be unusually rational for his day. Look at his highly unpopular progressive stances and this becomes obvious - they are typically stances that we would consider sane and moral today, but which were highly taboo at the time.

Personally, when it comes to clarity of thought, I've never found anyone who tops C.S. Peirce. He was unbelievably rational in his view of the world and of science and mathematics - he was scary. This is someone who recognized the fundamentally statistical nature of the sciences in the 1800's. He was able to see the broad philosophical implications of evolution in nearly as much clarity as many of your so-called "evolutionist" thinkers (can provide citation if requested).

C.S. Peirce is by far my greatest inspiration as an epistemic rationalist. Just look at what he did. The clarity and breadth of his thought and philosophy is incredible to me.