When I was looking to buy a CLS I initially wanted a 55 AMG. I test drove a few, and found one that I wanted. I was 27 years old at the time and the insurance quotes I was getting were ridiculous! The cheapest quote I got was just under £1600 per year, which I refuse to pay! I'm from Yorkshire! So if you are thinking of getting the 55 AMG I would inquire about insurance first.
Then I test drove a few 500's and found a 500, with the AMG factory styling pack, the newer 5.5 M273 engine, 2 years newer than the 55 AMG, £4k cheaper and in better condition. Insurance down to just under £500 per year! With 388hp it is still a quick car and on the public roads its hard to extract the extra hp from the 55 to be honest.
The 500 without the AMG tuning is very refined and so so smooth and quiet! Where as the 55 was a little stiffer and noisy,albeit good noise!
So I think you would be best to have a few test drives and decide what you prefer and what you could live with day to day. If looking at a 500 I would say look at the later 5.5 engined ones, such a noticeable difference in power and power delivery.
Also bear in mind that the running costs and price of parts for the AMG will be much higher. The only thing cheaper on the earlier 55's is the road tax.