This has been a very interesting thread, and I've been following it attentively seeing the different opinions, and it is interesting to see how everyone seems to be of the same opinion around these two specific cars.
Having been fortunate enough to own two E63 M6s, of vastly different character, as well as an E90 M3, I must admit it's heartening to see the love the old "V's" still have among enthusiasts, especially the V10.
While both the S65 and the S85 were architectural masterpieces, we all know the well documented issues both engines had with the obvious failings. I never had a day's issue with the V8, my first M6, other than an SMG solenoid that failed, was a solid and reliable car for the 50 odd thousand kms I had it. That car was visceral. The second one was a bit more problematic, but that was also because I think the car was very much unloved by its previous owner and the mileage (was about 150k when I bought it) was not on its side. I had to do throttle actuators and had a few other issues which were, luckily, not too expensive to fix, especially if you're handy with some spanners and have a multitude of size 10 sockets lying around (the car swallows those like a Swede swallows Molly at an underground rave).
Nah, they not that quick anymore, and you still have to ring its neck to get to the power band, but getting to the power band is half the fun, hearing that engine scream all the way through to 8300rpm is one of those rare aural experiences that will always give you goosebumps. A well tuned and maintained one will provide you years and miles of endless fun. Being the last normally aspirated V of any sort in the BMW stable, it has character, it has charm, it has a primal feel to it. Whether you marry it to the M5 or the M6 is a matter of choice. I have been fortunate to drive both and I feel the M6 is the more planted of the two due to its lower centre of gravity and shorter wheelbase. But the M5 is the more iconic, just because its an M5. The equivalent RS6 of the time was sporting a twin turbo V10, but the car had far less character than the M5 and M6. I drove an RS6 as a potential buy and it left me a bit cold.
If it were me, I'd be looking at a well sorted and maintained M6, from 2007 onwards, as there were some slight changes made in the bearing materials BMW used at the time. I spent a lot of time researching, and although the bearings are and always will be problematic, and preventative maintenance is a no brainer for any year, the 2007 models onwards for both the M5 and the M6 were less problematic than 2005 and 2006 models. So something to keep in mind.
On the M6, what you see is what you get. It came basically with NO additional features, and it didn't have the dynamic driver's seat with the automatic bolstering. The M5 had this as an option. Easiest way to check when looking at an M5 is there is a button in the centre console where the seat heating functionality is.
The SMG isn't as bad as people think if you know how to drive it properly. It still feels mechanical versus the modern 8 speed autos and the DCTs, and go all the way to level 5 on the shifting velocity (at your peril because the traction control HAS to be deactivated to get to it) and it becomes a very characterful mechanism to change cogs.
The M5 is a very special car, and far more popular than the M6, and the M6 had a premium cost over the M5, but that was for a reason. Now M6s are cheaper than M5s because of desire and appeal. I'd still love to own an M5, it's always been a halo car. And yes, just about everything modern is faster, hot hatches, M lites, but I can assure you NONE of those will give you the James May "fizz" like that V10 will.
You will spend a fortune in petrol. You will always worry if the engine will blow up. Every time it throws an error your poephol will suck you to the seat. Every time you service it you will pay for parts (especially if they are BMW OEM, M tax is a very real thing on these V10s), but Goldwagen as an example holds plenty of parts as used by BMW, but minus the BMW badge, at very competitive prices. If you're handy with a spanner, they aren't that difficult to maintain, with the right tools a throttle actuator replacement can take an hour. There's plenty of options to get new throttle actuators at decent prices (there's that dude in the UK that specialises in this and offers lifetime warranties on his work). There are enough good mechanics on this forum that can do bearings.
I miss having a V10, and I am really lusting after a decent M5 because I've done the M6. Their prices are pretty stable now. If you are looking at getting one, ensure it has traceable providence via invoices, contactable references, don't take anyone on their word that "all the big ticket items have been done", I think there's enough examples of this not being true. I just don't know if I want to get into ownership again having just purchased the 850 and having no space. But if the opportunity and the timing is right, I'd jump at it.
Ultimately my advice would if you can, why not. Everyone should have the privilege of experiencing one of the most iconic engines ever put in a production car.
