to start, the LS7 is a racing engine with a bit of reliability, whereas the merc engines will last for over a few million miles. Do note that the LS7 is a 7.0L and the 63's are 6.2L, and as many people in the US say "there is no replacement for displacement".
The LS7 is a phenomenal block and is MUCh more reliable than the previous LS engines, but nowhere near as reliable as an MB engine. As for the LS9, it's got forced induction (supercharger), as does the Audi RS4 (turbo), if that's the case i think the 2.6L RB26DETT and the 2JZGTE are far superior engines.
However if we are discussing mercs Vs chevys,.. perhaps we should compare flagship engines (NA and forced induction) as the LS7 is still the most powerful engine ever made by chevrolet.
2006 Corvette Z06 - 505hp, 7.0L (0-60=3.6) 1420kg, NA
1995 SL73AMG - 525hp, 7.3L (0-60=4.7) 2050kg, NA
1998 CLK GTR - 612hp, 6.9L (0-60=3.4) 1545kg, NA
1998 CLK LM - 600hp, 5.0L (0-60=?.?) 940kg, NA
1999 CLR - 600hp, 5.7L (0-60=?.?) 900kg, NA
2003 CL600 - 500hp, 5.5L (0-60=4.7) 2075kg, Biturbo
2004 CLK DTM - 582hp, 5.4L (0-60=3.8) 1748kg, supercharged
2006 CL65 AMG - 612hp, 6.0L (0-60=4.3) 2240kg, Biturbo
2007 CL600 - 517hp, 5.5L (0-60=4.5) 2185kg, Biturbo
2007 SLR722 - 650hp, 5.4L (0-60=3.5) 1768kg, supercharged
2008 Zonda C12R - 750hp, 6.0L (0-60=3.5) 1200kg approx, NA
As you can see, MB has been making plenty of power over the years with large capacity engines. just that the chassis they go in are heavy and are much more safer/advanced than any chevy ever made.
I love the vette but it's really just a track car with a cigarette lighter (smokers package) at a phenomenal price.