I'm looking at buying a Carrier or Trane furnace. The blower is going on my current furnace, so the A/C doesn't work too well. I have central Trane A/C, which works fine but there' not much air foing to the 2nd floor (split level). Any ideas on which is better (Carrier or Trane)? Can I mix & match, Trane A/C and Carrier furnace? What about two speed or variable speed? Thanks for your help.