Mercedes SL63 AMG vs SL65 AMG, these are the two most powerful SL models offered by Mercedes. SL63 is a V8 engine with 537 PS of power and 800 Nm of torque and needs only 4.3 seconds to reach 100 kph (62 mph) with 9.9 lt/km fuel consumption and 231 g/km CO2 emissions. On the other hand, SL65 AMG is a V12 engine with 630 PS of power and 1000 Nm of torque and needs only 4.0 seconds to 100 kph (62 mph) with 11.6 lt/km consumption and 270 g/km CO2 emissions.
However, SL65 AMG is a not very good choice, because the V12 engine is very powerful and the amount of power cannot be used efficiently. 630 PS of power and rear-wheel drive is not very good option! Also, the V12 engine is heavier than V8 and leads to manipulate weight distribution of the SL and may cause understeer. Lastly,V12 engine produces 1000 Nm of torque and this causes to increase the weight to transmission to cope with the excess amount of power.
Basically; SL63 AMG is cheaper, more economic and better packed :)