To third or fourth Bob's point about length: I've found some skis, like the Volkl AC4 or the RX8, to be especially length-sensitive. The 170 AC4 is a lively, quick fat carver, the 177 is a deliberate, ultra-smooth GS. The 165 RX8 is a relaxed, lively recreational carver that rocks bumps, while the 170 is sharper edged, more cross-like, and not nearly as much fun in bumps.
OTOH, I've found other skis (say the 6* Volkls in 168 and 175, or Head XRC 1200's in 163 and 170) to feel much less different than expected (beyond the obvious little more stabilty, little less quickness, in longer length).
Conclusion, besides demoing two lengths: Don't grab a bargin ski that isn't "quite" the length you wanted. May be a whole different beast.