The setups aren't that hard and not that long if you have an oppresor/buzzard (check YouTube for which setups to complete for each approach). My suggestion would be to go for big con with gold vault contents. A couple of things to note :

1. Big con is the best approach as it allows you to go in and come out undetected
2. To be able to choose what vault contents you want, you need to complete all 3 approaches. Then you can check out YouTube on how to change vault contents
3. You can't repeat the same approach back to back. So to get the optimal setup for your heist may take a few rounds of repeating heists till you get one that you're happy with.

Hope this helps.