I chose both the Wii and ps3 for the first (and to a lesser extent 2nd) party games.
My favorite Nintendo franchises: Zelda, Mario, Super Smash Bros., Mario Kart
My favorite Sony franchises: MGS, J&D, Ico/SotC, GoW
^All of those but MGS are 1st party. Sony's tendency to lock up awesome 2nd party deals (shown this generation through Ninja Theory's Heavenly Sword and Media Molecule's LittleBigPlanet, among others) was another reason I knew that the ps3 would be a good investment.
Also, I usually look at 1st party support first and foremost. 3rd party support can be a fickle thing, and it's never certain which console a developer's next game will end up on, whereas 1st party developers are always dedicated to specific platforms. And considering how robust Nintendo and Sony's in-house development teams are, you never have to worry about a lack of games. Especially when you throw 2nd party into the mix.
