I'm thinking the 1800 seems more up my alley, but that 1520 is tempting. I would be ok with either one, but even on 6S that 1520 seems kind of a low rpm. Sounds like it needs a pretty big project to take advantage of all that torque.
I run a Neu 1521/1Y which has basically the same Kv of the 1520 and I run 5S lipos now and 6S if I want to impress a few nitro heads. With 6S I would recommend that Kv and Castle got it right. The 1518 seems like a good choice for 5S lipos. This is good stuff coming from Castle, I'll be waiting for you guys to post feedbacks on how they perform.