Economic policy sounds like a dirty word in the context of Bitcoin, but as Jeff Garzik said, choosing a block size cap is unfortunately an economic policy that has to be chosen somehow. Enabling users to incentivise the voting process is an interesting tool to have in the toolbox, but I think it would be sensible to first observe how the miner-only voting system behaves on its own.
If, for example, the hashing majority tended to favour a move towards centralization (big blocks), user preferences could potentially hasten this move by further punishing marginal miners through reduced fees. On the other hand, if user preferences tended to oppose the preferences of miners, then such a system might function well in keeping a balance between usability and security (although it's not clear how this balance might change over time as the block subsidy drops).
In short, I think it's wise to keep it simple and implement one mechanism at a time.