Something like http://www.1-core.com/library/digital/fpga-logic-cells/
You can see there is a difference between diffirent Xilinx families and even bigger differences between the manafacturers.
On the bottom,
The architectures are very different, so it's difficult to compare them objectively. It should be understood that the results of such comparison have limited application.
It is usually the number of LUTs (and not the number of registers) that is a bottleneck for FPGA designs. From this perspective, it could be stated that one Virtex-5 slice can in theory substitute 8 Virtex-4 slices or 4 ALM's. But in real-world designs it is impossible to utilize all the resources.
For differences between chips of the same family you can look at the manufacturer site.
For example : http://www.xilinx.com/publications/prod_mktg/Spartan6_Product_Table.pdf