Our house is a 1995 tri-level home in Phoenix. It is 2196 square feet with an existing 5 ton single Trane unit. Two of the three bedrooms upstairs are 6+ degrees warmer than the rest of the house and they are south facing. Those rooms have new energy efficient windows - the rest of the house has 1995 dual panes. We have R30 insulation in the attic that was just blown in a few months ago. We have had several bids and have received very different advice. Each contractor states we should get a unit with variable speed. One contractor states the warm rooms have the registers too close to the door of the room and we should relocate them father away and install a 16 seer two stage Trane unit. The second contractor states we should add a second return in the warmer bedroom and install a 14 Seer Night and Day unit. The third contractor believes the American Standard Platinum 16 Seer two stage unit should equal out the heat issue and if not, then we address returns/moving registers. All of them are stating a 5 ton unit should be used again. All of them agree that the 18" return duct size is adequate and can be left as is. Only the one company suggests an additional 12" return in the warmer room to give additional air flow.

After reading many pages of this website forum I am really confused. It seems as though a two stage unit works best in humid areas and not in areas like mine with an average 5% humidity. Also it seems that 5 tons may be too large for the size home that I have at 2196 sq ft. No one did a load calculation (that I have been made aware of) so I will need to ask about that when I touch base next week. My main questions are:

Is 5 tons too much for a tri-level of my size?
Is a 2 stage unit a good idea for a hot, dry climate?
Should there be more return air than an 18" duct can provide?

We are leaning toward the American Standard, but I would appreciate any insight that anyone here would like to share. Thanks in advance for your time.