many times leaks can be repaired...why always selling all new equipment...

Efficiency depends on a lot of factors separate from SEER Ratings!

Do a little Research on Return Air Filter Sizing, etc.

Here is a link that might be useful: Importance of Filter Types Sizes Plus Data

Would you spend $300 more for a car that got 15mpg rather than 13mpg? You would save something like $500 the first year in gas.

Higher efficiency saves money in the long run. For me this would be a very simple decision, especially with the warranty increase.

I'm not a big lover of electrostatic filters. Pleated filters are easier to maintain. The newer 4" filters are once a year.

What size house? Two-story? brick? wood? windows? Shaded?
Any cooling issues? Any heating issues?

Gas furnaces? Will the blowers be replaced? Which blower models?

Lots of ways to look at it. What does an extended coverage plan cost for 5 years?

DOE probably has a worksheet on their web site to figure utility savings using your electricity rates.

15 mpg vs. 13 mpg over 10,000 miles would save $358 at $3.50/gallon. I only put about 5,000/year on my Grand Marquis and the pot holes don't phase it so I am keeping it. I do run my AC a lot so I have upwards from 17 SEER.

spend the extra for the 15 seer units. If you own the units for 10+ years you will surely see the savings.
THe 15 seer will cut your bill by alotvs. the old 10 seer.

Are you deciding between two condenser models? If so please post them. This will help determine if you are better off with one unit versus another.

In my opinion you should avoid builder's grade models. I think the XB13 falls into this category.

2 story house, but upstairs is normally closed off. When not, it has a separate 2-1/2 ton unit that we run and it is fine. Downstairs sq footage that these 2 units cover is about 3100. Brick, 14 windows (double pane), heavy shade most of the day. No heating or cooling issues. Gas furnaces/blowers will not be replaced unless problems are found during the AC install.

The biggest bonus in my opinion between a 15 seer and a 13 is the 15 seer units usually come with demand defrost which helps tremendously with unwarranted defrost cycles and with a txv valve at the coil instead of a piston type orifice which tremendously helps with humidity control and increases overall efficiency. These two items alone make the 15 seer pretty much the best bang for the buck of all seer ratings.

I replaced a perfectly good working builders grade system that was noisy and obnoxious for a 15 seer because of a zero interest option that ran for a limited time at work. It was a nice investment.