Q. Is it better to Daven Maariv before TZAAS with a minyan or after without a minyan even if you are makped to always daven after the TZAAZ?

A. The Mechaver (O.C. 235, 1) indicates that if the Tzibur daven Maariv early, while still day one should join them and read the Shema, after nightfall. Mishne Beruro (ibid. 12) states that; “Fortunate is the one who can recite Maariv in it’s proper time with a Tzibur”.
When this is not an option, (as when one travels) he quotes in Biur Halocho, the opinion of Ma’se Rav, that it is preferred to pray without a Minyan, at the proper time. However in Shaarei Tzion (ibid 16), he seems to agree with the Derech Hachaim that it is better to daven before nightfall with a Minyan, and to repeat the Shema at night, (as long as he davens after the Plag, and he recites Mincha before the Plag.

Horav Shloimo Miller’s Shlit”a opinion is that it depends on ones particular Minhag, when followed with consistency. Meaning that even on summer days, you are not Mekabel Shabbos early, and are constant in davening Maariv only after nightfall.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a