efficiency?
If you have a 3 ton load then a 3 ton it needs to be. To small and it runs to much. To big and it will not cool effectively. Get your contractor to do a Manual J calculation on the home then you will know that the unit is properly sized. Price difference between the sizes is not worth the risk.
Same applies to the furnace.
Yes the more efficient the less it costs to operate,,,,,,,, usually.
IMO installing anything less than a 12 is a mistake. Next year the minimum Seer for new construction will be 13. I base my thoughts on living in Phoenix, and cooling is ~ 2500 hours a year. For sure I would check to see what the cost difference is between your target and a 12 or 13. If your looking at the long term then check this site.
http://198.147.238.24/ac_calc/default.asp