Frame Rate
The Grand Theft Auto series has never been synonymous with a rock solid frame rate, something it generally seems to get a pass for. While GTA IV is smoother in general than it's predecessors, it can expectedly suffer during chaos. The Xbox 360 version seemed capable of achieving a smoother frame rate, peaking higher in certain circumstances, such as in confined spaces or during light traffic, but both versions seemed to regularly run at a similar clip.
Pop-In & Loading
With a huge, seamless streaming world, data must be constantly read from the disc. The PS3 version, with its required HDD install, loads missions a couple seconds faster and decreases pop-in. The Xbox 360 version can suffer from some very noticeable texture loading, as things like trees, vehicles and building signs will regularly pop-in as one tools about town. It can be distracting, especially during high speed chases, and the PS3 version isn't entirely immune from it, but it's not a game killing problem.
Graphics
The two versions are so close to identical, visually, that putting together a comparison was a challenge. Initially, it appeared that the PS3 suffered from more noticeable aliasing than its 360 counterpart, but after eyeballing the final retail copy, they appear to be on par. I noticed that the paint filter applied to cover some of the game's imperfections appeared a tiny bit muddier on the PS3 version, with night time scenes seeing more streakiness. Some of these minor complaints may be the result of our particular television settings and require the finest of fine tooth combs to assess.
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Final Thoughts
The good news for Grand Theft Auto fans is that whatever platform you have chosen as your own, GTA IV excels on both. They are almost evenly matched, feature for feature. Gamers will have to decide which is more important to them, a slightly more technically sound experience on the PlayStation 3 or downloadable content and Xbox Live integration on the Xbox 360.
If this were a personal decision, I'd have chosen the PlayStation 3 edition, mostly based on my preference for the DualShock controller and the performance edge. However, one thing that we have yet to experience is how well the PlayStation Network will handle the online portions of GTA IV, an extremely important consideration. I place less of an emphasis on features like Achievements and Gamerscore, but appreciate how well crafted and important these features are to others.
In the end, each version stands on its own, with the only downside to the differences being that those invested in their particularly backed console will expend so much effort focusing on something far less important than the game itself.