We aren't as good at being happy, but we might be better at improving the scope for future happiness. (E.g., maybe space colonization some day.)
This one becomes harder to justify if these critters are actually our intellectual superiors.
Other things matter (to us) besides happiness, and these critters' lives don't provide those things as well as ours do.
This answer may be irrelevant if you're only interested in utilitarian arguments.
My moral values are approximately utilitarian, but I would say that what I care about when thinking in utilitarian terms isn't happiness as such but those things of which happiness is a measure. (This doesn't require me not to care about happiness; happiness is in fact one of the things that makes us happy. If I were suddenly made much less happy about everything then that fact itself would be a source of unhappiness for me from then on.)
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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