Clomid causes an elevation of follicle stimulating hormone and luteinizing hormone (which signals the pituitary gland to produce testosterone). As a result, natural testosterone production is also increased which is the desired result post cycle. Arimidex is an aromatase inhibitor, meaning it inhibits the conversion of androgens to estrogen. This is an indirect way of getting your endogenous test production back to normal, not to mention, it is more expensive than clomid.
My 2 cents: Restoring endogenous test as quickly as possible is important for keeping gains post cycle. I would choose to use the most direct method available and save the arimidex for my next cycle. The combination of Arimidex during and clomid post cycle is a very potent one, & worth the sides IMO. Personally, I have had great success using Proviron @ 50mg/day during the last 3 weeks of my cycle (or the entire cycle @ 25 mg/day) & following with clomid.