תפלת ערבית is mentioned in the Tosefta Berakhot (3:6), and several places in the Talmud, e.g. Berakhot 4b, 6b, 26b, etc. תפלת מעריב does not appear in Hazal (at least none of the works included in the Bar-Ilan database).
However, it is a common term among Ashkenazim, and indeed, the term is found in quite early Ashkenazi works. These include the Mahzor Vitry (145) which emanates from Rashi's circles (11th cent.), Ra'avyah (Teshuvot Ubeiurei Sugyot 969) which dates from mid-12th century Germany, Sefer HaRokeah (Hilkhot Yom HaKippurim 218), which dates from late 12th century Germany, and multiple 13th-14th century works such as the Aggudah, and the Tashbets Kattan. It also appears in Tosafot to Megillah 4a (s.v. pasaq v'amar).
The fact that both the Mahzor Vitry in France, and Raavyah, in Germany use term, makes me suspect that the term precedes them, and perhaps goes back to the beginning of the Ashkenazi culture.
It should be noted, however, that some of these works may have been textually corrupted by later Ashkenazim, so the aforementioned survey isn't foolproof.
Indeed, we also find rather early Ashkenazi works using the Talmudic term, such as Sefer HaOrah (1:16) and Mahzor Vitry (1) which are both products of Rashi's school, and Raavan (140) of early 11th century Germany.
By the 14th century, we find Sephardi sources using the term, such as the Mnorat HaMaor (Kadmon).
It should be noted, that even if there is some significance to the change, that R. Mazuz S"T has noted that Rishonim were not particular with grammatical correctness, often referring, for example to the tallit g'dolah, as a tallit gadol. Accordingly, it seems very likely to me, that the Geonim and Sephardim mostly maintained the Talmudic term, while Ashkenazim often used their own very old term, and this was probably not a conscious point on the part of the Ashkenazim, and certainly not on the part of the Sephardim.
