It depends what you are doing with the car and what gen. If you have a 5th gen, then the only SS you can get is a guy in Europe who carries TTE parts, but he won't sell them unless you buy an x-trac drivetraine too, which is well over 20,000 usd used.
If you have a 6th gen, the parts from a 205 or SSII are supposed swap over. The flaw with the 205 SS is that is cast iron bits in the fig 8 ect which ads to unsprung weight. It does have lots of advantages though. There is a rally team in Russia that we are trying to get to make aluminum replacement parts for the 205 SS setup.
If you rally your car then Mc is the way to go. It is more reliable. TTE found that they gain 3 seconds on tarmac with the SS setup, but loose 8 seconds n the gravel. It was dropped from the 205 rally car. The same rally team making the other bits offers a Mc conversion for 205 rally cars.
Mc needs expensive bushing maintenance, and when they are worn out, I am told the car handles poorly. I used to want SS in my 185, but I would have to make a custom subframe to use the 205 bits. There is a rumor that one person has used SS inhis 185, but no details have surfaced.