They are two different types of AI, arimedex / anastrozole is a non-steroidal.
Aromasin / exemestane is a steroidal ai, increases IGF1, is a bit more aggressive, but is easier on lipids. Also helps to slightly increase testosterone levels.
The biggest difference you may notice is terrible vaginal dryness.
Proviron went out of style with Eisenhower battle jackets because there are better things to combat estrogen conversion (like aromatase inhibitors and SERMs.) But they don't do for the libido what proviron does.