The reason why this question probably wasn't answered previously is because square footage is not enough information to determine the size heat pump you will need. The economy and efficiency of a heat pump depends largely on the insulating properties of your walls, floors, ceilings, windows, etc. These properties vary with the different construction materials in your home. For example, the values for a brick home would differ from a wood frame home. Even the heat loss from the time your front door is left open in a day is calculated. I would suggest going to the library and checking out a book that explains this more throughly because it is quite complicated.
As for brands I guess thats relative to the observer. Trane is a popular brand but I have found Maratherm (a lesser priced brand) to be quite good. I personally wouldn't use GE.