Same top speed? The C350 will go 155mph? Might take you 5 minutes to get there.. I bet the C55 will go at least 180mph without the limiter.. I hardly consider that the same top speed.
maybe only 1 sec difference to 60.. but much different going to 100..
The C55 has everything upgraded, suspension, interior, engine, etc..
If they made a c500 it would be worth considering.. then just adding a supercharger.. but then you need suspension, sway bars, etc..
Get a C55 if you can afford it. Why not consider buying a used one with low miles?
good luck