In my opinion, the E24 M6 is the best looking BMW ever made...
I would agree with that.
In can't comment on the U.S., but in Europe M635csi's tend to be more collectible and up until quite recently were more expensive to buy than the M5, but prices for the M5 have really started firming up and are now similar in price to the M635Csi.
The M5 has a very small cult following but lately seems to be getting alot more press. IMHO the M635CSi appeals to a wider market as it is generally regarded as a very beautiful and rare car.
At the end of the day beauty is in the eye of the beholder, buy the car because you really want it not because you think it wil be worth more than another in five years.
If you are worried about being able to sell the car on when you have tired of it, both cars are highly desireable and collectible.
Best of luck with whatever you decide, or take old schools advice and buy both.
Im a sucker for the E28 M5. What can I say, I own two at the moment, and Im looking at acquiring a third, albeit a 1986 Euro Market with the M88/3.
Ive driven the E24 M6, one with a Dinan Stage III Stroker pumping out nearly 400hp and it was a monster in a tuxedo. The looks of the E24 cannot be beat. The E28 is a slightly ugly duckling, but that is part of its attraction to me, much like a Ferrari 412. The E24 is class and it will get attention today.
Lots of M6s out there, not a lot of M5s. So in terms of collectability:
Euro E28 M5
M6/M635CSi (US and Euro, US has a lot of fine fine cars, and they look great, even with the diving board bumpers).
US E28 M5
Im not very up to date on the M6 market, though it seems to carry a premium over the M5. Personally, Ill go for the M5, it just suits me better.
I've always loved the e24 styling(remember Moonlighting with Cybil Sheppard and Bruce Willis?) but, the E28 has that boxy 80's look that I just always have liked.I guess I can't think outside of the box?
For the most part e24's seem to have more value than e28's. A mint condition non-M e24 can be had for the same price as a decent NA spec e28 M5. Non-M e28's are sadly not worth anything. I think the e24 has a lot of appeal to the normal masses (be it M or non-M), while the e28 has no appeal outside of BMW enthusiast.
The E24 M6 holds more value, The E28 M5 is rarer and is finally climbing in value.
As for desirability and greatest value it has to be the Euro M635CSi.
This car is substantially different from a US Spec M6, and as it is more pure to the original design of the car as intended it will carry itself better in value. Good ones are expensive, and many have been better cared for then US Spec cars which often were leased, used and uncared for.
I presently have several M5s and M6s and two M635CSi's in my care, including the very first M6 brought to the US by BMW NA, by far the Euro cars attract the most attention, and the styling will never be disputed.
But my own S38 powered car is an E28 M5, for more practical to use and a better performer.
I own an 88 M5 and an '80 Euro 635CSi, style wise it is a non-contest, the 6er, particularly a euro, with small bumpers is a clear winner. The e28 is a great driving car particularly an M but takes a BMW lover to appreciate the style. Value wise it is clear the 6ers carry a premium. Low mileage M6 examples will command $25k up where a similar M5 (US spec) would be lucky to hit $15k. True Euro M5's (not m535's with the non M engine) are very rare and do demand pricing between the US M5 & M6 IF you can find one. The US M6 88/89 have the "world" bumpers are are asthetically much nicer then the earlier US models. The M635 body wise are preferable although I do not see that permium much in pricing.