Rondo and Melo are two guys that Houston has been rumored to covet over the past few moths. The front office of Houston has done a tremendous job of stock piling talent and will also have two lucrative expiring contracts in Lin and Asik to use as the building blocks in blockbuster deal if they choose.
If you're Houston, who would you rather go after? On one hand, Rondo has championship experience and a rugged, no-quit demeanor that Kevin McHale would love on a Houston roster who he has often called soft. On the other hand, the Harden/Carmelo combo would be hell for any team in the league to contain. Would be like Pierce/Allen on steroids.
Houston would ultimately have to give up less for Rondo, (Asik, Parsons, Beverley, and a 1st should get it done) and any deal for Carmelo would have to be a sign and trade scenario.