|I guess bottom line, what are the pros and cons for someone my size and ability for the 172s, and the 178s.
As wildcat said, the 178s will give you a little more float in fresh snow and stability at speed/in choppy snow. The con is that it's a little harder to maker short turns with them - because of the longer turn radius of the 178s, their increased swing weight/inertia and the fact that they'll be a little harder to flex because of the increased length.
the pros/cons of the 172 will be reversed.
The overall difference will not be huge, but will be noticeable. when people demo the same ski in different lengths, they often say "the 178 cm was too much ski for me" - i think this usually means the longer ski is too hard to flex for their weight, skiing speed and style.
You'll have to decide for yourself, but it sounds like the 172s will be a big improvement in float and stability. I also think that they're probably the better choice for most of your skiing, so you should keep em.
Finally not to nitpick - but for the price of new 8ks you could get two pairs of lightly used or new '07 skis with some careful shopping. Thus my two ski quiver comment, but I understand wanting new stuff for the beginning of the season.