Some background.
We live in Fresno, CA in a 2000 sq ft ranch style home. The house was built in the 1970s and has fairly decent insulation for the age of the home. All the windows in the home are currently single pane. The current AC unit is a 4 ton, 1990 era Night and Day Gas/Electric dual pack unit mounted on the roof (packaged units very common around here). The AC and the return are on the far East side of the house.
The previous owners converted the 300 sq ft garage, which is on the far west side of the home, into two bedrooms. They did place a 6x12" vent in each rooms and fed it by tapping into the main large flex duct that serves the main living area (also on the west side). The main living area is approx.. 800 sq ft and is cooled/heated by one large and very powerful 4 x 20" vent. The vents in the west end add-on rooms are approx 80' from where the A/C unit is located. Those rooms don't get much air and are much warmer or cooler than the rest of the house depending on the time of year.

We're now getting quotes for a new unit as the Night/Day model we have has developed a slow coolant leak. We've gotten three quotes so far from reputable contractors in town. Two of the quotes are from larger outfits and one of them is from a smaller contractor who was recommended to me by a retired HVAC installer. The quotes are as follows...

Contractor 1 (price lowest)
-Install new 4 Ton, 14 SEER Bryant 577CNWA48090N unit on roof
-Install new angle iron stand for unit
-Add return to one of the add-on rooms to increase airflow to that side of house
- Duct Test

Contractor 2 (+1250 after $400 Rheem/Ruud rebate)
-Install new 4 Ton Rheem 14 SEER unit on roof
-Install new angle iron stand for unit
-New Return for West end of the house (R6 Flex)
-New Thermostat (not really needed as we already have a new T-Stat)

Contractor 3 (+1200 after $400 Rheem/Ruud rebate)
-Install new 5 ton Ruud 14 Seer Achiever Series
-Install new angle iron stand for unit
-Remove current sheet metal on roof connecting unit to register/return and create straight downshot from unit to roof for less heat/cooling loss.
-Home run two new flex ducts from unit to add-on rooms. Cap off where previous ducts split off from main duct line.
-Add second vent to main living area so current vent doesn't have so much air hitting one spot and feels less like a tornado.
-Airtest ducts and seal as needed.

I'm leaning toward contractor 3 because he seemed to take his time and really look at the current configuration and how to best optimize it. I'm a little worried though that a 5 ton unit is going to be too large. The current 4 ton unit doesn't seem to have a problem keeping up and can cool the house (except the add-on rooms) from 90+ to 78 in a couple of hours.

Contractor 3's point is that because of the length of the duct we need the bigger unit to push enough air to sufficiently cool the add-on rooms. The first two contractors seemed to think that adding a return to one of those rooms would be okay with a new 4 ton unit. What do you think? I always read in these forums not to oversize the unit, so I'm a bit leery of a 5 ton unit. To further complicate matters we plan on changing to all dual pane high-e windows in the next year, so that really worries me a 5 ton is too big. Contractor 3 has said he will put in a 4 ton unit if I want; he’s just concerned I may call back in a year wanting to put fans in the ducts to push the air to the west rooms which would cost more than just getting the 5 ton unit up front.

I am however tempted by Contractor 1 based on the price. If the additional return in one of the west add-on rooms works it just seems like a great deal. However I did mention that Contractor 1 had spec'ed a Bryant unit to Contractor 3 and he said he used to sell Carrier/Bryant units but had several develop coil leaks within only two years and doesn't feel as though they're as reliable as the Rheem/Ruud units. Is the reason that the Rheem/Ruud quotes are higher is that they're really better units. I definitely don't want to put some serious $$ down and have to pony up more 24 months later because my coil is leaking.

Am I over analyzing this?

Sorry for the long post. I just always seem to see responses on here saying they need more info.

Thanks for any help/input.