I'm shopping for a water-to-air,open loop, heat pump. I received this today.

"I received the heat loss today which came to 55232 Btu’s and therefore at 70% sizing is 38662 which a 3 1/2 ton single speed or 4 ton two speed will do. I am pricing the 4 ton now."

Should I go 3 1/2 ton single or 4 ton dual?
Thanks

PS
If the single speed 3 1/2 T is $$$ cheaper, shouldn't I put that money into added insulation and reducing air leakage?

The single speed 3 1/2 T (042) is ratred for 34,900 Btuh (COP 5.2).
The dual speed 4 T (049) is rated at 34,000 Btuh (5.2) & 47,2000 (4.7).

We will use our propane Jotul Allagash (thermostatically controlled) (26,000 Btuh) to add heat on the really cold nights. (< 30%) rather than using the Aux heat strips.

Dancour:

A properly designed unit will maintain the design temperature 98-99% of the time. The other 1-2 percent will result in the temp being 1-3 degrees off the design temp. Usually this is not a problem as the house's heat mass will prevent you from noticing this.

Study after study shows that the vast majority of HVAC units are over sized. The reason is that dealers don't want call backs. Here in Florida you want a right sized unit and do not at all want an over sized unit due to humidity problem. Due to humidity issues I am very happy with my 2 speed Amana air source unit.

So to answer your question I would need to know the design temperatures used (i.e. did he use the recommended design temps or did he add a couple of degrees for safety already).

I really don't understand this: "therefore at 70% sizing..."

What is the basis for "70% sizing"?

Alex is right on.

Not sure where you are but in MI we shoot for about 95% heating with geo. 2 stage makes most sense with open loop or lop sided enviros (70% heating or cooling dominated) in my opinion. Can't think of an application where I thought propane was the best idea for auxiliary. In most cases here undiscounted electric space heaters cost less to run than LP. For emergencies (power outages) however, a propane furnace takes less generator to support it.
Good luck,
J