(Obviously I don't do much stone but I'll give this one a bash)
I'd choose quality and performance first out of the options given. I'd probably end up with a couple of options, so I'd go for the cheaper of the two. So price second perhaps.
Coverage and ease of application would be considered but I'm under the assumption that coverage of most would be competitive so similar with most? Might be wrong there. And application I'd say would be one of the least important factors when purchasing. Though I might regret that when applying.
I certainly wouldn't listen to a customer preference if I'm applying it and it's having my guarantee put to it.
And I'd take a similar view with an architect too, though if he's specified it and it's on his head, then so be it.
A major downfall of a sealer in my opinion would be one that needed doing more often than the rest. And perhaps one you need to apply several coats of compared to a competitors one or two coats.
Hope that helps.