Okay, first I'll start off by saying the topic title is perhaps a bit misleading. I hope to avoid, as much as is possible, the personal characteristics of these two people, but mostly their philosophies. Of course, their personal life can be seen as respective of their beliefs and how well they adhere to them.
First off, I would like to say I am a utilitarian. For those who have just taken Francione's definition of this, I am the school of Utilitarian that seeks to eliminate as much pain/discomfort for the most amount of sentient beings. I honestly believe that Francione is a utlitarian to, but fails to realise it because of his attempted exclusion from Singer. Afterall, he is seeking to eliminate the discomfort and pain of animals.
Having said this, to me, utilitarianism's end point would be something resembling Libertarian Egalitarianism. This would provide the least amount of pain and, consequently, probably the most amount of happiness. This would ultimately lead to Francione's theory of complete animal liberation as well.
I think people attack Utilitarianism because of Dr. Singer's particular branch of utilitarianism. As in the ARCO forum, where a similar discussion is currently taking place, Singer uses retrospection to make claims that some vivisection is justified. Of course this is only looking through hindsight. To me, as I said in the other forum, this wouldn't have even been allowed to begin with as it would be causing MORE pain to creatures unnecessarily. Therefore it's only justified AFTER the fact, and can never be justified DURING because it breaks utilitarian principles. And more often then not, vivisection is often futile, produces misleading results, or no results at all. This would NOT be a gamble I would take from my utilitarian stance.
Singer is slippery on the issue and I will admit that. And in his response he says, "Only if there is no other way." And there are other ways which makes vivisection doubly unjustified by utilitarian standards.
I think that is about as much as I will say on this issue for the moment. I no doubt have probably brought the ire of some. That's not my intent however. I think contentious issues SHOULD be brought into light (even though Francione eludes we should be careful bringing them up). Society hides these things under the veil of being taboo, and Francione seems to agree from his wording. Both individuals have their faults, but then nobody knows exactly how to achieve animal rights. It's a constantly evolving process. But as I said, I think it ultimately comes down to utilitarianism if you support animal rights.

