There are some critical differences between SLI and crossfire, which I believe demonstrate a fundamental difference in philosophy between AMD and NVIDIA. First, the basics, both technologies are pretty much a way to utilize more than one graphics card working in tandem in your PC to achieve next generation class performance that would not otherwise be available with the current technology due to power, thermal, or other limitations.
Think of it as if you could buy two Xbox 1s, put them together, and actually run your games at 1080p. They both work with anywhere from two to three and even four compatible cards in one system and perform their best at high resolutions with graphically demanding games. They also both dramatically increase the power consumption and heat output if your computer system without having a proportional impact on the performance you would get in games.
in fact another thing they share in common is that if the game doesn't have a profile implemented they may not improve performance at all. However, with that said, if you're looking for an experienced that simply isn't available today, by other means like butter smooth 4k or 1080p surround gaming, then you're likely doing what the solution was designed for.
When you want to join graphics cards, compatibility is the key. This is differentiation point number one for AMD. NVidia allows mix and matching vendors and clock speed. You can SLI EVGA reference card with an Asus overclocked one. They require you to use exactly the same graphics processor with the same memory configuration. AMD on the other hand, gives more options. You can mix vendors, clock speed, RAM amounts, and even the graphics processor, as long as the cards are within the same architectural family. For example, a 7970 graphics card can Crossfire with a 7950 with 3GB of RAM, and/or even an R9 280X with 6GB of RAM based on that same GPU.
Differentiation point number two for AMD is cost. NVidia requires SLI certification for your cards to recognize your system as compatible and activate their wonder twin powers. For that, NVidia requires the qualifying PCI Express slots to run at 8x minimum, even though these days a 4x PCIe Gen 3 slot is plenty for pretty much any graphics card. NVidia also required the board manufacturer to pay a licensing fee. You can find AMD crossfire compatibility on product pages for like business class boards. Anything goes for AMD, as long as you got couple PCI Express 16x physical slots.
Differentiation point number three is something that actually is still a work in progress for AMD since the beginning of dual GPU configuration and SLI; there has been some kind of connector attaching the cards. With AMD R9 290 series cards and possibly future ones as well, all communication between the cards done over the PCI Express bus. It is a higher bandwidth cleaner looking solution to the problem on how these cards can synchronize data with each other at high speed when they are working together.
Finally, differentiation point number four, is that AMD allows their cards, their low-end cards to run in crossfire with the onboard graphics. Although, this solution still needs some work to be something that I would really recommend.
I guess that is the philosophical difference between AMD and NVIDIA. NVidia locks things down, which gives the entire control over the consistency of the user’s experience but AMD offers more flexibility and choice, even if some of these options are not the best thing ever, at least you can pick them.
