This is a question that has come up in the past on this Forum. I found out the differences between the two. I was cheap so I bought the RTC3201. Then I got to thinking, so I splurged and bought the STC1186. There are two main differences that are important for tuning.
1. The STC lever arm is about 1/8 inch shorter (centers spark better on cap)
2. The STC spring is much stiffer (improves the advance curve, maybe)

The STC1186 is the correct part, and worth the extra cost, in my book.

Problem with the RTC is that is does not align the spark well between cap and rotor on this distributor. Even though I ran one on my truck, the spark occurs on the last bit of trailing edges of the rotor and cap. You can check this with a new cap, run it around a couple miles, then see where the spark mark occurs. It will shift to

D1 vacuum advance modules
be more centered with the STC, since the shorter lever changes the relative position of the cap and rotor, but not the reluctor. If you change one for the other, the timing will need to be adjusted. If you use the RTC, the cap and rotor will wear out faster. The STC1186 says Made in England too, and feels stouter than the cheap one. I am hoping this one will last longer than the cheap unit I put in last time and the diaphram popped.

Hope this is helpful. The D1s are getting old enough that there aren't as many posts as 11 years ago.