I have a customer home right now in which they are faced with this same thing. Home call for (3.5) ton load and they want 2 stage XR16. So would I will have to go with a (4) ton in this case. I am in the process of laying out new duct system for proper supply and return for the (4) ton system to cover what they want. Other upgrades they can't afford right now do to having replace the system. So either way you need to ensure duct work, line set etc... Is also properly sized for the system to work right and not be noisey.
I had even talked with them about upgrading insualtion and windows to lower the load on the home to get it to
(3) tons? Again due to $$$ issues with the customer they can't do and propely want to once we are complete.
So what I would do is ask the dealer you choose to run the numbers with adding add. Insulation to attic (most cost effective) and see how much load would be reduced. By going this way you might break even with installing more insulation and being able to down size the equipment to cost around the same amount? I would just ask them to ensure you get the most bang for the buck?