Despite criticism regarding job loss if certain benchmarks are not met on aggregates like Metacritic and Gamerankings, I've always been a fan of locating comprehensive critical information quickly and easily.
Ultimately, Metacritic better serves my needs for a number of reasons. I prefer the layout and as a gamer feel it offers me more intel on a specific title than Gamerankings. Metacritic offers easy access to a game trailer, game description, critic reviews, user reviews, the side bar compares the scores of recent releases for a platform. Gamerankings, on the other hand, lacks these functions but provides a clearer understanding of a game's reception in terms of the big picture. It offers the game's annual rank for its platform, all time rank for its platform, multiplatform rank for the year, and all time multiplatform rank.
Another argument in favor of Gamerankings is its game exclusivity. Rather than focusing on multiple entertainment mediums, the site solely tracks one making compiling more efficient. I personally desire to have a slew of resources at my fingertips instantly, fermenting a bias towards Gamerankings, but for those of us who enjoy taking apart the details, Gamerankings is definitely the better choice.
Metacritic caters more to my needs but I acknowledge the opposing faction's views.