The way I see it, there are problems with any of these cars as an investment.
The are limited production, this means that parts will become an issue at some point, and getting bespoke parts made is more costly as there are less other others likely to want the part - so little cost reduction on volume - says the guy with perhaps the rarest of all parts wise, that 4200 manual box :S
That's good and bad, as broken expensive parts are likely to lead to less cars. We've already seen some more or less abandoned 4200 manuals based on broken gear cables..
They aren't cheap to run, and sticking one in a garage is fine as long as you keep it maintained, which obviously costs money.
If you want to invest in something for approx the cost of a Maserati Coupe, that won't cost a fortune to run, but will appreciate, I'd be looking at a Rolex sports watch or something, much less maintenance, and easier to store!
The other option (dare I suggest it) is to not bother maintaining it (within reason obviously). Providing the car still looks good overall, and works, it'll sell. And if the market goes up and up, cars at the bottom end of the market will still appreciate and continue to sell - less money in = more money to make on an average car..
Look at the Ferrari 308.
10 or so years ago, you could pick up a GT4 for £8-9k and a GTB / S for £10k and up.
Now they are £45-50k+
Who's going to tell me that all the no service history, high mileage piles of Italian rust are all perfect now to make them worth that much..?
Then you have the price differentiation between a regular, or facelift 4200, or even a 3200 and a GS is too big at the moment. The GS is seen as the halo car, not the 3200. That will take a lot to change.
The next issue is that each car has it's merits, so there isn't really a straight forward answer to which to buy (perhaps the 4200 being the least distinctive of the trio is why it's the lowest valued one?)
Also, on the whole, these are largely forgotten cars. The Porsche GTx, and older air cooled cars are poster cars. The GTx cars are perceived as being amongst the best at what they do. The Maserati Coupes are neither - perhaps the GS could be a poster car I guess..
So... after all that.
Sod it. Buy a car to enjoy, not for investment, and you won't go wrong. Buy for investment and you'll either never use it, or you'll be full of regret if it either doesn't appreciate, something expensive needs fixed, or if the bubble bursts and your stuck with it.