You say you want the best value for your dollar, but neither one of these will give necessarily give you money back for money spent. Duct cleaning is beneficial if you believe you have a problem with your indoor air quality, such as alot of dust or if you have problems with allergies, etc. Otherwise they may not even need cleaning. Here's a link that'll explain lots to you about it.
http://www.epa.gov/iaq/pubs/airduct.html
A blower door test is more of an energy audit, it will tell you where you are needing help. It will uncover inadequate insulation, leaks around the home (in the attic, ducting, windows, doors, walls, etc.) It's only if you carry through with the repairs that the blower test reveals, that you save money and recoupe the investment. Here's a link that'll tell you all about them:
http://www.infiltec.com/inf-bd.htm
If you live in an older home and you are paying high heating and cooling bills, then I would say the blower door test will save you more money in the long run. But, if you're paying large amounts of medical bills due to allergies then perhaps the duct cleaning would be for you.
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