If you intend to become a mileage junkie then do the upgrade instead of the free ticket unless you are going to use the free ticket to substitute for a last minute purchase at an outrageous price.
Put another way: The 80,000 miles gets you one free Biz RT. The 80,000 miles plus about $1200 gets you two Biz RTs, and you earn about 24,000 miles for the travel (once you make Premier Executive), for a net cost of $1200 plus 56,000 miles. Also the two Biz RTs are now paid fares instead of freebies, and the flight miles count toward your retaining Premier or Premier Exec status for 2002.