I would personally do a center support and shorter slats (or a center support with a plywood deck if that's the preference) for a Queen bed just as I would for a King. Even my daughter's Ikea queen bed is built that way to help distribute the load evenly. I also personally prefer slats because of how they breath, but there's nothing wrong with plywood as long as the specific mattress is ok to use on a solid deck as Brian mentions.
--
The most expensive tool is the one you buy "cheaply" and often...