What are the differences in the sl600 and the sl55 other than the v8 vs. v12?

Why get an sl600 when the sl55 is just as fast(according to Road and Track 0-60 is exactly the same)? Top speed seems to be the same as well.

 

Thre are many things that are diffferent.
Most noticeably are the different engine types you mentioned.
The AMG version use a V8 with supercharger, and the SL600 goes with V12, aided by two turbochargers.
And when speaking of Mercedes, V12 is the KING.
On paper, Mercedes claims the horsepower figures for both cars. But various dyno tests has proven that the V12 engine was well under-rated.
The acceleration figures seems to be the similar, but the AMG 55's gearbox is better-suited for stop-light to stop-light times. (such as AMG Speed-shift and better ratio)
AMG offers (factory option) top speed up to 300km/h or 186mph in Europe for both cars.
THe SL55 is designed to become more of a sports car, rather than an Autobahn crusier.
SL55 -- for people who enjoys driving
SL600 -- for people who has the $$$