Suzi's advice sounds fine.
If you do decide to go with one of the larger local firms, we've had good experience with Bryant, who we retain for routine semiannual inspection and maintenance. We switched to them from another local firm after discovering one winter night (when our furnace's computer circuit board blew its mind and quit working around 2 a.m.) that their idea of "24x7" service meant "our night answering service will take your call and give it to us when we get to work in the morning."