If you'd asked me this question a few years ago, I would've said Activision all the way. However, the tables have turned recently.
EA used to be the evil all-consuming conglomerate with no regards for innovation and not a single quality product in their lineup. However, they have recently started buying up talented companies, allowing them to keep the staffing that made them successful, and pretty much letting these studios do what they want with EA's considerable resources. As a result, EA has become a purveyor of some of the best brands on the market.
Activision, on the other hand, used to be a decent company that believed in taking risks and putting out quality products. Unfortunately, that's completely flipped, as well. Now, they buy up talented studios, slowly gut them or force all of the original staff out, and flood the market with increasingly low-quality sequels, driving what was once a decent franchise into the ground (Guitar Hero, Tony Hawk, etc.). They've also developed a habit of cutting projects (such as Ghostbusters or Brutal Legend) that they "can't exploit on a yearly basis" while keeping relatively worthless franchises (Spyro, the Ice Age tie-ins) that can be used to make a quick buck by essentially pumping out the same game every year. Also, they've been doing horrible things to Blizzard, forcing the company to squeeze its customers for every last cent.
So, in closing, Activision has made me very sad recently, whereas EA has gotten its game together and decided to compete with quality products rather than quantity or underhanded business practices (I know EA's not completely honest, but at least they're not trying to buy the rights to sports anymore just to kill competition). So, I side with EA, and proudly.
