I think it depends on the cars each one competeing against for example.

in the E63 vs M5: I'd take the Benz just for the fact it's like the perfect sleeper car if you shave of the E63 bade and AMG badge of it looks like a E550. provided your not paying attention to wheels(AMG wheels) the E63 and M5 almost within a few grand in pricing so it's a buyer's pick pretty much.

in the the C63 vs M3: It depends if you want a track car or small sedan with some serious zip. if I was buying it for me I'd go with the C63. the new m3 isn't really doing much for me. to me the C63 is hotter. I haven't heard pricing on the C63 but i expect it around upper 50's or low 60s range. the unoffical M3 price(new gen) is 60 grand.

in the SLK55 vs Z4M:I'd take z4M hardtop.(plus you can pocket the 10 grand in the difference from z4m hardtop to a SLK55 throw some wicked dubs,tires on it and a bitching sound system.

then if you got serious cash:

Alpina B7 vs S63: I'd take the S63. just the 7 series doesn't do much for me. just the S series just looks hotter to me.