Both models are excellent speakers. The AR-2A grew out of the original AR-2, and added a third driver, which is basically a super-tweeter. The rap on both of these models seems to be the angled dual cone tweeter, which is a good driver(s) but fails to receive particularly high acclaim from most users. The 2A model, with the added dome (super) tweeter, tends to cover for any deficiencies from the dual driver with regards to high frequencies. The AR-2A is intended to be used in a horizontal orientation for best dispersion. The cloth woofers are extraordinary, and the 2A was often supplied with a very cool grille cloth.
The mid driver (with mesh cover) in the 2ax is a definite performance improvement, and allows the speaker to be placed in horizontal or vertical orientation. It was released in two basic versions: early one had cloth woofer and 1-1/2" dome tweeter, while the later one had a foam woofer and 3/4" dome tweeter.
Yep, you should own a pair of AR's. Read up some more on these two models and then go inspect them in person. Either way, you will not make a bad choice.