The main difference between the roadster and the coupe (apart for the retractable roof, of course) is that the open top SLS will not use the coupe's gull-wing doors, but will most likely benefit from conventional doors, or SLR-like ones.
As for the engine, not much is known so far. Its coupe cousin will boast an AMG 6.2l V8 powerplant that generates 571 horsepower (420 kW) at 6,800 rpm and a maximum torque of 650 Nm at 4,750 rpm and it's likely that will be fitted on the roadster as well. Still, rumor goes the soft-top may
As we reported a week ago, Mercedes also has plans for an electric-powered SLS AMG powertrain. According to the manufacturer, the EV version of the supercar is faster than the future Tesla Model S, getting to sixty in under 4 seconds and producing about the same power as the 6.2l V8. It will be introduced on the coupe and perhaps will get the chance to see it on the roadster as well.
Whereas the coupe is expected to make its public debut at the 2010 Geneva Auto Show, the convertible may arrive a bit later. By the looks of it though, the car looks nearly complete, so probably 2010 will also witness its entrance to the market as well.
Future spyshots and details are poised to surface, so keep an eye out for the developments.