I work in the HVAC field for a local company, and I install a lot of Lennox, or Bryant units, I put a lennox in at my dads place, because that is what I trust for him. Either of those are great units. Another thing to look at is the contractor. Is he fly by night guy or a guy thats been in business for years. Check and see whether his quote, takes into account a new plenum or any tin they may need to put in or cut out. Are you getting a high or mid efficiency furnace? If you have any questions about things, let me know!
Denis