Since a proper installation is a key issue, I would ask about the initial dehydration process and vacuum test. Such as, how many microns do you pull the system down to and how long did it hold steady at 500 microns or below. Was Nitrogen introduced to dry out and pressure test a system after the first vacuum and before the second vacuum, or is it only a one step process? Was Nitrogen gas flowing through the pipes during the brazing process of joining the pipes together? It makes a huge difference in the elimination of scale inside the pipe which can clog or affect the metering valve.

Some extra time in the beginning can save you a world of hurt down the road.

I actually have used this company here Airforce Air Conditioning Company.

They are great! Very honest and all of their service people are background checked and screened thoroughly.

Here is a link that might be useful: Air Conditioning Contractor

I would ask to see the specs for the equipment. Does it meet or exceed the total btu's, the latent as well as the sensible btu requirements. In today's world a "3 ton" may not actually be or even close to a nominal 3 ton value. The only way to be sure what you are getting performance wise is by asking for this info. IF not then the system will not deliver the comfort you are expecting. IF you system seems marginal now then ask for a load calc (Manual J) be performed. IF you do this be sure to specify the cooling and heating temps YOU want in order to again ensure your comfort. Also remember that if the two firms use different equipment the prices will vary. Be sure to ask about their warranty. Does it include weekends, holidays, etc. One may offer it as part of the price the other may not. One price may include an additional 'manufacturers warranty' while the other may not so be sure to ask IF one is included or not. Good luck with your choice