Well, I am using MMA SE for over three years. It is very easy to search, chat and get a quick response on SE sites. On SE sites, it is very highly likely you will get an answer to your question even before asking (relevant search). I am also using MaplePrime for more than 10 years, the website is always very slow. It happens time to time that old Q&A are lost or quite hard to search.
Apart from all of this, a Maple SE site will attract more crowd and will help to build a strong literature.
Am I making any sense?
Forgot to mention. Because of MMA SE, I am learning Mathematica quickly, otherwise, it was a lost cause.