How proficient of a skier are you? If you've been skiing for a while I would go with the 173s. I recently got longer skis than I used to have and love it. It will feel a little odd for a run or two, and then you will get a feel for the new skis and be able to charge harder because they will be more stable for you. At the same time, if you're newer to skiing it will be harder to drive a big ski than it will a small one.
If you can find a free or cheap ski demo, perhaps try demoing a longer ski before you buy one.