This is a long debated topic lately and has been one of the most controversial in the gaming community. However in my opinion each one has its place and to be honest, are quite honestly too different to compare. Battlefield 3, originally designed for computer platform, is an amazing first person shooter designed to encompass the entire war strategy game type by creating huge fields of game play which allow for more tactical, large scale game strategies. Modern Warfare 3, originally designed for console systems, (and most Call of Dutys for that matter) are geared more toward close proximity, direct contact fire fights which creates a game type which generally consists of much quicker skirmishes. While the spawn system has never been perfected in the Call of Duty series, the advantage to it is that you are not simply starting at one side of the map every time, and are always right in the action no matter what. Many people consistently complain about this however lets face it, it does make the game more interesting and prevents games from being prolonged more than necessary. In Battlefield 3 you are also able to enjoy the use of vehicles which adds a new element to warfare strategies, however while you face an imperfect spawn system in Modern Warfare, you are often times helpless against enemy vehicles such as tanks and aircraft in Battlefield 3. This is often aggravating to the people who are in it for the person to person ground based fighting.
Simply put, each has its advantages and disadvantages, however at the same time I do not feel that these two games can so easily be compared against one another considering they were originally designed for completely different platforms and focus on two completely different elements of warfare strategy games.
While most people are taking a firm holding on one side, I am simply torn between them both. They are both great games and would recommend both to anyone. I am currently playing both and have about an even amount of time dedicated to each. The reason I play both is simply because they feel to me like two completely different types of games even though they are both first person shooters. Hopefully eventually the heat will die down on this topic because I get really tired of hearing both sides bashing on one another when I am playing both and enjoy them equally as much. I think most true gamers would agree with that and I don't know why this bothers me so much but I thought I would throw out my two cents and see what people thought. Leave me a comment if you disagree but back it up with a real reason.