Being familiar with GTA universe, my personal preference is Fallout 3, only recently just experiencing this hit RPG. I find the post apocolyptic theme far more captivating than the crime/mafia style of GTA.

Both games have their pros and cons, but the concept of each GTA is more than conventional. A bit repetitive, you might say. Protaginists in the big city, gradually rising through the ranks of the criminal underworld. You begin to notice that the only motivation is when the protaginists are left for dead or betrayed by one of many commonly antagonists.

On the other hand, you are enabled to decide which missions you wish to undertake, and your relationships with various characters are based upon the choices you make. You are given an open world enviornment (some areas must be unlocked thoughout the course of the game) in which many buildings are accessible. The ability to use vehicles simulates a working city with pedestrians that obey traffic signals.

Fallout 3 is similar to GTA IV in many ways, also having a massive open world enviornment with accessable buildings and the ability to make relations with various characters.

What makes Fallout 3 stand out is its post apocalyptic theme unlike any other, taking place in the year 2277. "200 years after the nuclear apocalypse that devastated the game's world in a future where international conflicts culminated in a Sino-American war in the second half of the 21st century". You embark on a journey through the relentless and unmerceful wastelands of Washington D.C. in search for your father, all the while making a reputation for yourself. The choices you make hold great consequence, affecting your karma and how characters interact with you. The game has a broad variety of interactable objects, including weapons. V.A.T.S. (Vault-Tec Assisted Targeting System) gives you the ability to target a certain limb of an enemy and deal enough damage to cripple or even mutilate that limb depending on the maximum damage and/or condition of your weapon. A great addition is the party system. You may have a maximum of three, consisting of you, your dog (Dogmeat), and a non-playable character.

I could add so much more information to this post concerning the pros and cons of the two games, but all in all, both are great and deeply satisfying.