You say you have trouble cooling your 2200 sq. ft. home with a 5 ton as it is. Start there. Have a competent company look at your ductwork and perform a Manual J load calculation to size the equipment properly. The issue may lie in a deficient ductwork system. Is this an attic system? I wouldn't put so much emphasis on the equipment but rather on finding a good contractor to address your needs. A 5-ton unit really should be able to cool your home unless it's all windows with no insulation (to put into perspective: I have 3.5 tons doing ~3500 sq. ft. just fine--including finished basement, so really around 2800 sq. ft. or so--in through the hot/humid MD/DC/VA summers; however, my home is from the 80s). You say the ductwork is in poor shape, so start here. You want an efficient system that has good ductwork that won't leak the conditioned air. The fact that one came up with a 4 ton and one came up with a 5 ton sends up a bit of a red flag. How was humidity control with the 5 ton? A sign of oversized equipment is less humidity removal and shorter cycles, which costs more money. I would recommend a variable-speed blower and a 13/14 SEER outdoor unit. We can go back and forth with what brand is the best, but from what I gather, you have other issues to tackle first.

All three contractors looked at the ductwork, and all three felt that it wasn't the best, although they focused more on duct placement than sizing. Prior to my owning it, the house went through three conversions - originally, it was a swamp cooler (I believe), then went to heat only, and then to heat and ac. So the ductwork is somewhat piecemeal, and they reused outlets that were placed for a heat only system... so no question that we're starting from a disadvantage. One contractor really focused on the return air situation and felt that was the biggest issue.

As for being oversized, we don't have anything like the VA/DC humidity here in SoCal, but I definitely do not see a short cycle problem - on the contrary, the compressor runs continuously, trying (and often failing) to keep up with the heat. As I mentioned, the current compressor is 5ton (most likely 10 SEER, given that it dates from the early 90s), and two of the three contractors suggested replacing with a 5ton system. I need to ask the third why he thinks 4ton (without much duct work) would be appropriate.

Your sizing issue may be caused by the bad ductwork. Have the contractors search for leaks and make sure they are sealed, then perform a load calculation to determine sizing. That's really the best way to size the system. Some of the air may be leaking if the ductwork is real bad. Do you know how well insulated the home is? If it's a real leaky home due to its age, that may be the reason for the high heat load. Just speculation from my computer. :-)

That's a good point - some of the problem is likely insulation - i.e. the house has very,very little attic insulation. I'm planning to have someone blow in insulation once the HVAC work is done, and I'm sure that will help. As far as leaking elsewhere, well, due to the house's age, while I don't specifically know where it is leaking, I'm sure it does.

Jeff08 is 100% correct, even a small duct leak can cause an ac to work very hard and appear to barely cool. The contractors should be able to evaluate your duct work and offer suggestions for up grades. If you pay for a manual J load calculation make sure you actually get one, some have a copied set of calculations, your name is typed in you are feed some techno jargon and charged. Good luck Iggie

So.. just to get back to the original question (yes, the ducts need work; yes, insulation will help; yes, it would be great to get a full man J)...

Can anyone give me any insight on the equipment itself? I can't figure out if I'm even comparing apples and apples here.

Here is a linked page that might help you on the duct system.
- udarrell

Here is a link that might be useful: Correct Sizing of Residential Duct System & AC