Smaller units will require a smaller backup genset to run one at a time if you want to do that. (Inverter units are already very good with gensets, btw.) As others have pointed out, efficiency may be higher with the 1:1 systems. Some mini splits can cool and heat simultaneously transferring heat from one room to another, but these are rarely seen in residential situations because they are more expensive. It may only be available in systems with more indoor units. There is one downside to heat/cool units if you only need cooling. Complexity is increased meaning more things to break. I don't know if that is significant. I had a reversal valve fail under warranty which cost the installer, but not me. Note that if you have oil heat, the heat pumps may be a less expensive way to heat. Two 1:1 systems means another compressor unit sitting outside. If I had all 1:1 systems, I'd have seven compressors sitting on pads :-( You should know that the better mini splits have inverter-controlled compressors and variable speed blowers. This combination increases efficiency and comfort by stabilizing temperature and improving humidity control. Now consider this. A single unit that can throttle back to 30% of 6000 BTU can run at a 2000 BTU minimum. If you have an outdoor unit that can run 12,000 BTU your minimum becomes 4000 BTU. I believe that the smallest multi Mitsubishi compressor unit is 20,000 BTU so in this example, its minimum might be 6,000 BTU so you gain little from variable speed unless you are running both units at the same time. This would be enough to make me go with 1:1 in your situation despite "slight" increased cost and another unit to maintain. In addition to those arguments, if the price is similar, but you are running one compressor at a time most of the time, you get more longevity for the compressors.

There is what is probably an under appreciated advantage to oversizing variable speed heat pump equipment. The systems run at a higher efficiency at part load. I am finding this true for boilers as well as heat pumps. I expect it is true for furnaces too. Any of this equipment probably "saturates" the heat exchangers as they approach top operating capacity. In fact, HPMS equipment has a sometimes quotes overcapacity that is seen for a few minutes after start-up. Inefficiency could also come from increased resistance to refrigerant flow at higher rates. I am no engineer so my reasoning is all hand-waving. I wonder what happens in multi units when all indoor heads are operating vs. only one in this respect. If I knew, I might operate my system differently sometimes.

I see. You will be fortunate if the commercial outfit can follow through if they are exposed to a wider variety of equipment. I guess that they will be installing, in contrast, residential equipment. The commercial stuff does some neat things, but it costly. It does variable refrigerant flow (VRF). That allows you to move heat from one place in the building to another. In a home, for example, if you have one room warm, another other cool, and you need a little dehumidifying, you could run one unit in cool or dehumidify mode and another in heat with the same compressor. The outdoor coil could be put out of the circuit. I think that as a schoolboy I was in Peru at least once. I grew up, however, on the other side of Fly Creek from there in a deep river valley in Chenango County. I tell people that it would be the bottom of another finger lake if the South end was stoppered up like some of the valleys to the West. It is a nice geologic perspective from a couple of angles. As you drive down them, you get a good feel of how deep some of those lakes are. In addition, how easily that particular and other valleys could have been lakes as you look at the river beside you and the hilltops high above. I guess the same thing could be said to the North and East where the glaciers scraped out valleys, but they look less like fingers.

I do not know the tonnage of the unit, but it is A/C only and provides cooling to the living room and kitchen areas via two roof ducts. This system doesn't cool the bedrooms at all. It's also a freon-based system and can no longer be legally serviced or recharged. Ok first off, R22 Freon units can be legally serviced and recharged. Someone is trying to sell you on misinformation. Obviously a 25 year old unit is going to be difficult to get parts for if not impossible, especially a Freon based unit. You'll pay a higher price for the refrigerant but it's still readily available. I just bought a jug of R22 yesterday for a leaking coil replacement job I am doing later today. Residential units that contain less than 50 lbs of refrigerant are not under any mandatory replacement schedule. An 1800 sq. ft. structure in your climate is not going to contain anywhere close to 50 lbs of refrigerant. ICP is more or less Carrier, with a different sticker on the side of the unit. These companies are both owned by United Technologies Corp, a large multi-national conglomerate. I only say this to give you some understanding. I've seen similar homes like the Rummer model here in Houston area. That structure the design of it has a good number of challenges to make air conditioning work properly. The difference in cost between these two HVAC contractors that quoted the job could be due to experience in problems encountered. Your climate in Oregon is not overly extreme, but very humid. What that might mean? The HVAC contractor that quoted you $10K higher, may better know the challenges involved with your structure. OR maybe they don't have the experience with that particular structure and they are hesitant / want to be sure they don't risk their company's good reputation on a possible job gone bad. Good reputations can be ruined on one job gone wrong. The company offering this for $10K less may be inexperienced or they may be experienced to know what they can do the work for because they have done these structures in the past. So what does your gut tell you? You could get two more companies involved and see where they wind up quoting you. I can tell you this: When I quote a job, I quote it for what I feel the job is worth. I don't quote a job out of fear I am quoting it higher than some other company. I am a Licensed pro with a salty long history. If you call me - you get me and my ways. There is no magical science behind why there is such a disparity in price. It may be related to skill of the technicians on staff. This is an easy way to drop price is to hire a bunch of kids with no experience. But that isn't to say the $10K higher priced quote is any different in that regard. But you don't keep skilled technicians without paying them and you don't pay them without collecting for it down stream. With that said, if both these companies lied to you about being able to service an older R22 Freon unit, I think I would steer clear of that. If they are going to lie about that, what else will they lie about? R22 Freon Requirements CIRCA 2019 EPA 50 pounds or more rule. With that said, you should fix any refrigerant leaks, even though the rule as stated doesn't require you to do so. I service the Katy, Texas area.