They all have a unique character.
I guess you need to ask a few questions:
1. What do you plan to use the car for?
If is your only car, then do you need the additional rear seat space?
The Cayman is probably easier down country lanes, since it is smaller.
The C4S is very sure footed.
2. Do you want to have a soft top?
I personally don't like them, so I would go for a Cayman or the C4S.
The main point of the Boxster is that it is a roadster and has a soft top. If you don't intend to use it, then why bother having it.
The coupe body is stiffer and more secure, so why compromise if you don't want the wind in you hair.
3. Power is addictive and you always want more.
It seems that there are no end of posts saying that the Boxster or Cayman should have the 911 engine, or be an RS version, or have turbos. If you want a 911 engine, you'll have to buy a 911.
4. Do you like the new interiors, or prefer the 996 style?
The C4S does not look like the 997/987/C7S. Do you like the newer steering wheel and dash?
5. Do you plan to keep the car for a long time?
6. Do you care about the depreciation?
7. Do you car about image?
The 911 is a 911, and rightly or wrongly has a stronger image.
No doubt work colleagues and friends will be more impressed with the C4S.
I think the Cayman is a great proposition, although I am not convinced about the styling, but it is not a 911.
Ultimately there is only one way to tell.
Look at the all in the metal, test drive them, then go with what you want.
Do not go with what you think you should get, since you will always want the other one, and changing your mind is getting increasingly expensive. Also do not go with what other people tell you to get, since it is your money, so pick what makes you happy.
Edit:
I would also add, go with what your initial gut feel told you to get. You seem to want a C4S, so don't be put off by one dealer, go to another OPC or independent.