All cars are the same too. They all have wheels and an engine so why get a Caddy? Just get a Kia, they are all the same.
Actually, some of the better known brands have a good reputation because they produce better equipment. But installation is VERY important. A good unit poorly installed will likely give you nothing but grief.
Having you for his helper seems strange, unless you have training in HVAC. I would keep looking. Seems all he has to offer is low price. Every time I buy something cheap, I wish I had spent more money on something better.
If you are happy with your utility bills and comfort, I would stick with the heat pump. And I would go more efficient than 13 SEER. A more efficient unit will save you money. But you should see how much extra it costs before you decide.
Remember, Air Conditioning begins with AIR.