Lucky board. It would almost always be the expenditure of choice for a ftp. For someone at your power, lucky board could very well boost you thousands of power for the 1k coupon.

On a side note, if you are ready to use the medium thread right now (done with first 3 ranks), DO NOT USE THEM YET. Battle armor rebate is coming in 4 days, use them THEN, so you can get MORE threads. (Unless you are like me... on rank 5.... that's not going to be enough threads so... meh...)