Make sure your heat gain matches the capacity of the unit. A 1.5 ton unit may not have a 1.5 ton sensible capacity.
Yes, a new one will be more efficient than your old one.
Don't forget about that old a coil and furnace....
Proper Prior Planning Prevents Piss Poor Performance
"There is hardly anything in the world that some man cannot make a little worse and sell a little cheaper, and the people who consider price only are this man's lawful prey. It's unwise to pay too little.
When you pay too much, you lose a little money -- that is all. When you pay too little, you may lose everything, because the thing you bought was incapable of doing the thing it was bought to do.
The common law of business balance prohibits paying a little and getting a lot -- it can't be done. If you deal with the lowest bidder, it is well to add something for the risk you run. And if you do that, you will have enough to pay for something better."