Altera is the major rival to Xilinx for the FPGA market. To compete with the Spartan-3 series, Altera developed the Cyclone II FPGA line (they’re up to Cyclone V, and have apparently skipped to Cyclone 10 now.). Search the web for the low end Cyclone II EP2C5 and compare its features to those of the Xilinx Spartan-3E XC3100E. Some of the features you might look at include gate count, number of LUTs (and number of inputs to each LUT), numbers of multipliers, clock speed if available, etc.) Note that Altera’s name for a LUT is a Logic Element (LE), which doesn’t quite correspond to a LUT but it’s close.