I would suggest playing all the free weekly and monthly tournaments, the credits are minimal, but free none the less.
I would also suggest playing ready gos and match play challenges(for credits). With this said I would look at some of the results from the ready gos and your current skill level to see which ones if any would be best for you to enter. Ready gos are not for the faint at heart, as for most it is hard to knock out profit on a consistent basis.
Secondly, when looking at playing MPC, you must be careful when selecting opponents. This might mean that you set up the game so that you can look at a players stats once they enter. I would be looking for anomolies, like a player with great MPC record against all tiers, more specifically a player who consistenly beats players of higher tiers.
After looking at your profile, I would suggest waiting on all of these. IMO, you need much more experience before doing both. A hack with 3 ranked rounds, IMO, is not even close to where you need to be to compete in both these credit earning adventures.
Stick to the freebies and try some offers for now, and when the time is right, go back to my first suggestions.
Cheers,
b0gey