Hi, there are loads of threads discussing what is the best thing to go for, a 172 or 182. They usually end up in arguments as it's only a persons opinion at the end of the day.
I'll give you a few points.
The 172 spec is largely the same as the 182 apart from the following:
182's have ESP, cruise and speed limiters.
However, 2003 on 172's have ESP and 53 on 172's have ESP and the speed/cruise controls.
182 has a 200 cell sports cat, 4-2-1 SS manifold which mainly give rise to the performance differences.
182's also run with 205/40 16's on PE2's which gives a different feel to the ride and makes it feel slightly more planted.
There were two variants to the 172's. 172 and 172 Cup. The Cup was the light weight option with Speedline Turinis which are a slightly wider wheel giving a wider track. Also, no ABS and cloth interior. Also came in mondial blue and some later models in iceberg. The Cups have a slightly larger rear spoiler, which does look alot better. Cups also have pretty much no extras, e.g. xenons, auto lights. The same spec is present on the 182 Cup.
With the 182 there were a few models. 182, 182 Cup, 182 with Cup packs or 182 Trophy.
182 = same as 172, just the styling and performance differences.
182 Cup = same as 172 Cup with styling and performance differences.
182 with Cup packs is the 182 spec e.g. xenons, auto lights etc with the Cup options. Cup options included grey wheels, cup rear spoiler, stiffened shocks and springs. Also a bit lower.
I won't go into the Trophy.
This is largley the differences. Personally I'd go for either a 172 or 182 with Cup packs.
172's are also about 1-2 mpg better than 182's overall combined as well.