Personally I would say get neither. Sure, they're the fastest cars other than the Furai, but they're simply too fast to be anywhere near useful.
The SS model specifically if double upgraded will do some weird aquaplaning shit when you accelerate meaning it loses traction on the ground and you can't actually steer properly. The SS model also has next to no customisation as you can't change the carbon fibre body nor the colour of the rims, so you're basically limited to the slither of colour around the skirting.
If you want a fast car, consider literally anything else. McLaren P1, Pagani Zonda C12, Porsche Carrera GT, Mercedes McLaren SLR, Audi R8 Plus, and the Ferrari F430 are some extremely solid choices.