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IMO the make, features and efficiency are most important while you choose
a model but the installation is most important when choosing a
dealer/installer. Since most dealers sell limited models and brands, You
will need to find the optimal point between these two factors. I would
say that if a dealer can answer your question satisfactorally, then use
him.

Absolutely right. They are both important. I would just add that
reliability/longevity is also important for brand choice.

We did this about a year ago, and had about six or seven companies quote the
job (I know conventional wisdom is 3, but we just kept going until we felt
we had the right combination). You want a good installer, sure. But if the
equipment fails in 5 years the quality of the response you get will have
more to do with the manufacturer than the installer. And of course, a
reliable brand will be less likely to fail in the first place. The
installer we chose spoke authoritatively about the reliability/quality of
the equipment he was recommending, which gave us confidence in both.

Good luck.