Service/maintenance plans, home warranties, & upkeep

[Daniel Brown]

I was just wondering what the general consensus is on maintaining HVAC systems. My wife and I own two single-family homes that we rent, and have just entered into a one-year service agreement with a reputable HVAC company in the area for about $300 apiece. The agreement covers two service calls a year, cleaning of the A/C and heating units each once a year, and a 15% discount on repairs and maintenance. Is this preferable to purchasing a home warranty (all the HVAC systems are new in the past 3 years) that has a set service call fee, but (theoretically) covers most of the maintenance, for a higher annual price ($552/yr)? What do most landlords of single-family homes do for routine maintenance and service? Are we being too proactive (ie, would it be better to just do one inspection/cleaning a year)?