Which is better when inviting people to Allah: saying "This religion is great" or "This Islam is great"?

There is no difference in using "this religion" and "Islam," as both are correct. Allah Almighty said: (And whoever desires a religion other than Islam, it will never be accepted from him, and in the Hereafter he will be among the losers) and (Indeed, the religion in the sight of Allah is Islam). The word "religion" often requires a genitive construct, such as "the religion of Islam."