Replacing a 25yr HVAC Unit. Could use some advice on replacement & ducts.

Living in central NC, my 27 year old, 2ton Trane gas pack for my first floor finally decided that was it and won't start up anymore so I'm now looking to replace it. I've received a few quotes on Rheem, American Standard, and Trane units and I've decided to go with the Rheem (any experiences?).

2 ton vs 2.5 ton: The installer said my sq ft is right in between the 2 ton and 2.5 ton sizing. Since there is about a month in the winter and a month in the summer where the unit cant keep up with heating and cooling demands (inside tracks with temp outside), I opted for the 2.5 ton. With that they are replacing the return duct with a 16" flex. Nothing mentioned about the return grill (currently 20"x20" with 1" MERV 8 or 11 filters).

Duct Work: I'm debating right now whether to replace ductwork instead of sealing & cleaning, but I've decided I will look into it now but that will be a decision for later (no cost savings since it's a completely separate job). I believe most of my ducts are flex, but will verify at the installation. What advantages are there going from flex duct to rigid duct? Every HVAC company I've talked to seems to think I'm crazy for requesting quotes on rigid ducts.