in what area you wish to compare the two. the 2 products are no long just a simulator already. they have packed in so many features like assertion, coverage, etc in it.
when compare with cadence LDV and NCverilog , synopsys VCS7 is better.
read deepchip.com reviews
about both LDV , NCverilog and VCS. VCS is better than other simulation from cadence.
but globally accepted simulation tool is modelsim