First of all, do you need to use them for text search indexing? GIN and GiST are index specialized for some data types. If you need to index simple char or integer values then the normal B-Tree index is the best.
Anyway, PostgreSQL documentation has a chapter on GIST and one on GIN, where you can find more info.
And, last but not least, the best way to find which is best is to generate sample data (as much as you need to be a real scenario) and then create a GIST index, measuring how much time is needed to create the index, insert a new value, execute a sample query. Then drop the index and do the same with a GIN index. Compare the values and you will have the answer you need, based on your data.
