I am looking to replace my 12-year old Goodman Performance 90 gas furnace (cracked heat exchanger). I've narrowed my choice to either a Carrier Comfort 92 or Trane XR-90. As the secondary heat exchanger on my old unit failed, I am particularly interested in the material used for the secondary heat exchanger on the two units- Serpentuff polypropolene-coated for the Carrier and Stainless Steel for the Trane. Should I be concerned that the polypropolene-coated heat exchanger might go the way of my current system? Or will it perform comparably to the stainless steel?

I'm also considering replacing my AC at the same time (same age as the furnace). Am I being premature in replacing the AC or does it make sense to take care of both at the same time? If I do decide to replace the AC, should the coil material be a consideration (copper in Carrier Performance 13 vs. aluminum in Trane XL14i)? Also, I believe the Trane uses a scroll compressor; any idea whether the Carrier does also?

Finally, any other info I should consider in deciding between the two? Consumer reports gives Trane the nod regarding reliability; is the Carrier a satisfactory alternative? Any comments on the above questions would be greatly appreciated.