At your weight, I'd say the 177 is the best choice in the AC-20. Yes, you could choose the AC-30 in a 170 if you wanted to, but shorter is only better for a few things. The better balanced a skier you are, the easier it is to be comfortable on a shortish ski. Note that many PSIA types choose rather short skis as their work tools.
On the more or less forgiving thing, here is what I mean. A stiffer ski like the AC-30 gives you some rewards like stability, grip, energy (sometimes) dampening (sometimes) etc. Like everything else in life and ski choices, you give something up to get those things. In general, that higher end ski will not initiate as easliy and will grip so hard that it can be reluctant to smear and release at the end of the turn. This is fine on smooth packed snow and nice dry snow. However, this can be tiring in grippy moguls or mixed and broken snow. A more agressive ski will respond very energetically to tail pressure. A high level skier may apply tail pressure in order to elicit something from the ski. That skier knows what he/she is doing and knows what is coming. Someone else OTH, may not be ready for the response and end up in the backseat or get drop kicked toward next week.
Ultimately, choosing a high level or mid level ski is dependant upon what you expect. I ski so many different skis every year that I find myself feeling comfortable on most all of them. Despite my 190# weight and skiing level (~~L-8), I find myself more comfortable on skis like the Legend 8K or the Nordie Nitrous. Those are not the most aggro skis within their categories but that's what I have come to appreciate the most.