Aromasin has a dose of many mgs/day [not week]. It is simply more expensive in many cases. But arimidex from a pharmacy is costly. Side effects? I don't know... but you are talking way more of it and large doses in general may imply some sides, making, in general, smaller dose amounts attractive.
I did a lot of PMs on another BB where a guy did aromasin after reading the hype, and had poor results. He then switched to arimidex, but I have not heard how that worked for him, but there is no doubt in my mind about its effectiveness.