For the first time in history, Argentina will play a home tie on a surface other than Clay. This was originally made to cancel out the great advantage Nadal would have on the surface, but since Nadal unfortunatly pulled out of this final with injury, this might have little relevance. However Nalbandian has indoor hard courts as his favorite surface, and Del Potro prefers it over clay, so this might have been the best choice in any case.
Spain has made the very bad decision (I think), not to call up Juan Carlos Ferrero for this tie. He has a very decent H2H against Nalbandian, and I believe that he would pose the biggest threat to him, other than perhaps Ferrer. Ferrer has been in pretty poor shape, but was the one player that came the closest to beating Nalbandian during his Madrid - Paris double last year, being two points away from victory.
Del Potro is not quite in the same form that led him to four consecutive titles during the summer, but should be motivated enough for this tie.
Feliciano Lopez is the spanyard that is most comfortable on this fast surface. Should he be picked to play a tie, he might threaten Nalbandian and Del Potro, however I doubt that he will emerge victorious against either of them.
For Spain, how well Ferrer plays as well as the doubles tie will be crucial. Nalbandian was not picked for the doubles however, and the pairing of Acasuso and Calleri should be seen as underdogs against Verdasco/Lopez.
All in all I believe that even if Ferrer finds his late 2007 form, Argentina will have a big advantage here, and I can see them winning comfortably, 5-0 or 4-1.
Source : http://www.bbc.co.uk/dna/606/A43781637