Thanks Rob, that doco explains clearly how functions work with EL. I must admit though I'm not use to this odd way variables and functions are treated. I'm use to other programming languages like VB and FORTRAN. I can see why it's done this way but it does require more time to think and make sure the right settings and variable types are used. Call me old fashioned but I prefer the normal way as it avoids confusion in very complex programs. I suppose I'll get used to it.
Anyway, after reading the doco, the answer to my own question is either will work since I'm not referring to any historical user defined variables within the function, but Simple is better as it uses less resources.
Also, if I wanted to pass a numeric variable as a parameter I need to define it as NumericSeries in order to refer to past values but still can define the function as Simple (I think).