AC residential units are nearly all identical. Most use the scroll compressor by Copeland, however, some are coming in from Asian countries. When you look at the air handlers they all look like they are built by the same company with just a different label. What is different is usually the control circuits. Basic units (SEER 14) are almost interchangeable. When they go bad a generic unit will work fine. When you get into the SEER 16+ units is where the proprietary controls will vary and variable speed compressors come into play. SEER 18 as you listed all show two speed, etc.
The outside condenser units will vary slightly. First is Bryant and Carrier are the same. Bryant is an upscale version where the housing is a little more robust with a better power coat finish over Carrier. Bryant may have a sound deadening blanket over the compressor for quiet operation and if the compressor sits next to a patio you want the quietest unit you can afford. Many companies now list a sound rating. Trane outside compressor unit cover is good but one small thing that it takes some maintenanc. Its all lover design causes rain water not to get to the coils to wash them down since they will accumulate dust, dirt pollen, etc. No big deal just something to know.
Most AC units now uses an ECM (electronic controlled motor) motor for the air handler and condenser fans. They use 240v, convert it to DC for the DC motor and run very efficient. It's one way to get the SEER rating up one a notch. They are very easy to control the fan speed with a 1-12v signal input. When they go bad they are very expensive to replace. My Carrier unit fan when bad past one year but under the five years parts warranty. It still cost me $275 in labor. Most of these motors are made by Genteq, a division of Regal-Beloit Corporation. GE was first to patent this motor but GE sold off their small motor division. As you stated, lifetime warranty on the compressor but he labor will be very expensive. It might be cheaper to buy a new compressor unit.
A SEER 16 unit is well worth the price as compared to the minimum SEER 14. Once you get over SEER 16 is where the price really jumps. as you seem to have mentioned. Regardless of what you choose, they all are about the same in quality and your electric bill will drop.
