Hi Max.
The 182 is the newer of the two.
They are slightly different styling features too. Most noticably the alloys are different, and with the 182, you get the twin exhaust coming out of the bumper at the back.
The 182 also has an extra 10bhp than the 172. I believe this is achieved through a different manifold.
Inside, I think they are exactly the same.
I've got a 172, and my mate has got a 182. They are both very similar cars. The only difference between my mates and mine is the fact that its got 10bhp more, the alloys, the twin exhausts and he has cruise control.
Hope this helps!