Advice on custom blocks

Hi,

I am planning on building a custom block for fault insertion problem in simulink and I'm having trouble deciding what kind of block I should create. ie S-function, Subsystem, Matlab function

Here are its main aims.

1.To be able to be complied for xPC target into the real time workshop environment

2.To be able to use all forms of data types (different fault types)

3. Hopefully to change its data type output middle of simulation

4. To be able to have simple error checking or the parameters set and also to be robust to user error.

I'm leaning towards an S function would be best but i never created one so I'm not sure. Thanks

Paul