SEER stands for Seasonal Energy Efficiency Ratio: cooling delivered over a season divided by the electricity used to deliver it. Higher is better. In 2023 the industry moved to SEER2, which tests equipment against higher external static pressure, closer to what a real duct system imposes. The rating scale shifted down slightly as a result, so a SEER2 number and an old SEER number aren't directly comparable.
The practical question isn't which number is bigger, it's whether the upgrade pays back. That depends on how many hours the system runs, and in the Rio Grande Valley it runs a lot. A cooling season here is long enough that efficiency differences which would be academic further north turn into real money.
The rough arithmetic: take your summer cooling cost, and the percentage difference in efficiency between the two units is roughly the percentage you'd save on that portion of the bill. Divide the price difference between the systems by the annual saving and you have a payback period in years. If it comes out shorter than the equipment's expected life by a comfortable margin, the upgrade makes sense. If it doesn't, put the money into ductwork instead.
That last point matters more than the brochure suggests. A high-efficiency system on leaking, undersized duct will not deliver its rated performance, because the rating assumes the air actually reaches the room. Efficiency you can't deliver isn't efficiency.
Two other things worth knowing. Variable-speed and two-stage equipment usually feels better than its rating suggests, because longer, slower run times remove more humidity, which matters here. And any efficiency claim depends on correct charge and airflow at installation, which is why a commissioning report with real readings is worth more than a sticker on the cabinet.