The 172C has a CSTN display, which is short for color super-twist nematic, an LCD technology developed by Sharp Electronics Corporation.
Unlike TFT (like on the 182C), CSTN is based on a passive matrix, which is less expensive to produce. The original CSTN displays developed in the early 90's suffered from slow response times and ghosting. Recent advances in the technology, however, have made CSTN a viable alternative to active-matrix display (like on the 182C).
New CSTN displays offer 100ms response times, a 140 degree viewing angle, and high-quality color rivaling TFT displays (like on the 182C)- all at about half the cost.
However, the TFT technology (182C) provides the best resolution of all the flat-panel techniques, amd is daylight viewable, but it is the most expensive.
TFT has an active matrix type of flat-panel display in which the screen is refreshed more frequently than in conventional passive-matrix displays. The two terms, active matrix and TFT, are often used interchangeably.
So, the way I see it, the 172C has a smaller, less expensive display which may be more difficult to see in day light. The 172C display needs an adjustable backlight to allow you to manually alter the display so you can see it in sunlight. The big advantage of the 182C is that the dislplay is daylight viewable, meaning you can see it at all times without having to make any adjustments.
I haven't seen the 172C display side by side with the 182C, but my guess is that the 182C will be better, but you really need to see them in sunlight to show how much better the 182C is. The 172C has 256 colours versus 16 colours on the 182C but I don't think this is much of an advantage for the 172C because the charts don't use more than 16 colours, anyway.
Having said all of this, the CSTN technology has come on in leaps and bounds in recent years, so they might be a lot closer to the 182C.
Another difference I noticed was that the 172C has 10,000 tracklog points compared to 2,500 on the 182C. This isn't a real big deal, but it will mean you will use up the tracklog memory on the 182C quicker, and so have to clear it more often. Again, this isn't a big deal unless you do a lot of high resolution tracking.
Bob F.