Tough call, I mean you just started skiing and you say you're progressing quickly and youre already jumping to powder orientated skis, do you have enough experience, technique to ski powder? Because these skis won't ski the same on groomers and you might find it challenging. If you're really set on these due to their price, go for the 186s, however skis closer to your height might be better but you say you learn fast so a few days/weeks on them and you'll be fine. The 176s will be too small for you.
Look for something else if you won't be skiing powder but piste. The line prophet skis are really easy and fun skis to ride on, they respond so well. I got a pair of 179s and I'm also 5'11" ish , and they shred so hard, and I can ski great with them in powder but you might have to backseat a little in the really deep stuff.