I am in MN and I have a 27ft round pool. It holds 18,500 gallons. So it has 572sq ft of surface area. Looking this up, it looks like it would need about about 200BTUs. Looking at Heaters, the most common are Heyward and Raypak. Do any northern people have opinions on this? Looks like both have a heater just at the 200K BTUs and then next up is about 250k. So would a person go with either brand or another, and would i go right at the level of btu or go 1 step higher.