I have a 12 year old 3.5 ton 12 seer Carrier electric heat pump. I'm told that parts are rusting (coils?), it has a small refrigerant leak and basically it's got about 1-2 years left. I have a quote for a 3.5 ton 14 seer Carrier Performance series ($8,000, installed) and I'm being told that the new Carrier will be much more efficient and cost less to run (50% less.) Does 2 seers really make that much difference? Is the Infinity a better product?