Xilinx and Altera fundamentally do the same thing to an almost silly extent. There are a handful of fundamental building blocks that make up an FPGA, and in every case Xilinx's implementation differs from Altera, but in trivial ways. Altera have an Adaptive Logic module whereas Xilinx have a Configurable Logic Block. They're really just look up tables.

So where the rubber meets the road is how good the tooling is and how well they execute on their tapeout. Xilinx has been much better executing their plans since Intel bought Altera.