If you just want to get into fck, we recommend the pure flavour
Flavours of fck use the same language, syntax, etc. The difference is what comes with the language by default. One of the aims of fck was to have a multipurpose language. This means that there are several flavours that have built-in functions, types, etc. based on the flavour you're using.
For example, the counting flavour (for maths) includes several additional types such as matrices, and functions such as the digamma function.