M maintenance is not terrible. The oil changes are more expensive because the oil is more expensive. The valve adjustment thing - going 30k miles between them isn't terrible on the S54. If you can't do it yourself, just wait until the valve cover starts leaking and have the mechanic do it then.
Seriously, though, the driving experience is different. Do not read that as "better." The 3.0 will have softer suspension and, if you require, the option of an automatic transmission. The M...not so much. I prefer the M, obviously, but I also recognize that not everyone likes to row their own gears and rattle fillings on rough pavement like I do. Some folks like to cruise around in quiet comfort with some power on tap and eye-catching looks. The 3.0 will get you that.
As was said, no changes between 07 and 08, so go for a low mile 07 and see if you can score a deal. Your issue is that the prices will be higher now than they will in, say, three months.
Try and get one with some warranty left if the reliability scares you.