LUTs vs Logic Elements/Cells

The way a design fits in a device is more dependent on the synthesis and fitter software than it is on the device itself. I use both Xilinx and Altera every day and I promise you that at this point in time Altera is kicking Xilinx's tail in the software arena.

If you want some rough idea of fit comparisons, see the opencores tests that were run:

http://www.altera.com/products/devices/stratix-fpgas/stratix-iii/overview/architecture/performance/st3-opencores.html

So you could pick a Xilinx device that looks larger than an Altera device and most likely your compile times will increase and you will actually fit less. We struggle with this now. We have Xilinx designs that take almost a whole day to compile. We have equivalently sized Altera designs that compile in an hour.

Jake