Ask each contractor for literature on their recommendations and compare this data to make an educated decision.
Consider things like warranty, performance metrics, energy star ratings, etc....
Try not to let cost be the determining factor & see if their prices are negotiable after loading yourself with information that may be useful in the negotiation process. If there is a sizeable cost difference between a few particular units, ask why this may be.
Keep in mind there are several models within the same brand of units and make sure you don't under or over size a unit.
Bigger is not always better when it comes to HVAC.