Recently, I had the "pleasure" of producing three separate prototype solutions for a problem our company has related to reports and presenting them to my bosses. Each had its share of advantages and disadvantages in terms of development time, performance, scalability (time between start of project and being able to begin producing reports), ability for clients to modify these reports, etc.
My bosses are notoriously known for their inability to make decisions of this nature, so I wasn't expecting wondrous things. So after explaining the advantages and disadvantages of each prototype, I asked which prototype they would have wanted. They were undecided (surprise!). My bosses were not going to commit to a decision without a suggestion from us as to which would be better (aka, you decide and if it fails, it'll get pinned on you).
So I decided to take a different approach: I asked which of these priorities is more important (development time, performance, scalability, etc.). Again, they were somehow able to evade point of my question by saying that all these priorities were important.
Finally, what finally worked was a strategy I adopted in my attempts to play off their inability to choose priorities. I made a table which have priorities on the x-axis and prototypes on the y-axis. For each priority and prototype, I gave it a number value from 1 to 10 based on how "ideal" it is in that respect, 10 obviously being the best (don't make 10 the worst for any particular aspect or you only risk to confuse yourself later).
Once you have this, you pick the strength of one prototype and you pit it against the strength of another prototype. So in other words, you begin forming questions like: "Which is more important? Development time or performance?" "Which is more important? Performance or scalability?"
At this point, really stretch these questions to get them an idea of perspective:
"Which is more important? We finish in 6 months time or that the client has to wait half as long for reports?"
"Which is more important? The client has to wait half as long for reports or that we can begin to produce reports within the end of the week?"
At this point, my bosses could handle making decisions in bite-sized pieces and could give a good indication of which prototype they preferred based on this information.
The inverse works as well, pitting weakness against weakness: "Which is worse? It takes us longer than 6 months time to finish or that the client complains about the time it takes to create a report?"
In your case, we're talking about features, not priorties, but I think the same can apply here. "Which would you prefer? Making it available on iPhone or enhancing performance?" Each question is easy to answer and can give you insight as to which features are more important. In your case, I would assign 1 point for each feature chosen in this way in order to be able to effectively gauge one feature with respect to another.