I've had different contractors tell me different things... probably based mostly on what they'd like to sell me! Some (especially the Trane dealers) said one or more of their 2-stage offerings is "most popular", by far. Others said they rarely sell the 2-stage equipment these days since the IRS tax credits expired. I tend to believe the latter. I have a variety of quotes on 2-stage and the less expensive 15 SEER units and I've searched through the county's online permit site to get a feel for who's installed which units in my neighborhood over the past year. While the site doesn't reveal model numbers, it does show the cost estimate for the job. Looks to me like the higher cost 2-stage installs have been few and far between over the past year, regardless of what the dealers would like me to believe!
Checking dealer permits also gives me an idea of which of the one-man shops are in this business for beer money only!