I would give very serious thought before chopping in the MC for a Cayman. As great as the Cayman is, whilst it is easier to live with, the overall ownership experience is less special.
Having to ask the question what to buy instead of the MC almost gives you the answer. For the price these cars are commanding, there is nothing comparable in my opinion.
I like to change my main car every couple of years just to try something else. I'm now however into my third year of MC ownership and the fifth year of Z4 ownership but can see nothing that I could afford to buy/run that excites me as much as the MC still does so the car stays.
I understand the opinion about the M6 being wrong on a number of levels but in my opinion, this comes close to being a worthy car to move to if you can accept its shortcomings. It certainly is a bargain buy as ot sufferes from the perception of being expensive to run. My question is "compared to what?"
My local dealer says expect at least 10mpg less from the M6 than the MC. Whilst that is a figure that I would feel, it's not unreasonable for the overall package you get. Servicing is not cheap but again, for a car like this it doesn't sound like bend you over the table and take this without lubricant plus give me a body part expensive either. The thing to remember the M6 is bloody cheap to buy.
Regarding holding onto it as it will be a sought after classic in 10 years. If you can afford to wrap it up and tuck it away then fine but in 10 years i would hope there would be something as special as the MC is now. I would rather drive mine into the ground now and have a great time doing it. Life is not a rehearsal and that blast accross country now is something I'd rather have as a memory in 10 years than a dream of what the car could be used for.
All of this is just using lots of words to say simply, there are lots of other cars, some quite nice but in my opinion, nothing a special as the MC for the giveaway bargains they are today. Did I see a nice one on hear with mileage a little leggy at close to £15K? Double that and there is nothing else I'd want but for £24K, I'd rather stick with what you've got and put the extra into something else other.
Pesonally, I've put what I would have spent on a newer car into a classic car. This gives lots of fun and an alternative to the MC which makes the MC feel even more special when I come back to it.