They both have very similar horsepower, yet completely different engines and prices.

No, I cannot afford either of these. I am just curious which performs better and why their prices are so different.

The SLR Mclaren can go 209mph and the body is made out of carbon fiber so it is also lighter than the SL65 AMG. The SL65 AMG is limited to go up to 155mph because it was not built to go past 160mph. The SL65 goes from 0-60 in about 4.4 seconds while the Mclaren does it in 3.9 seconds. The SLR is a supercar built in limited numbers while the SL65 AMG is mass produced and a sportscar-not supercar.