It depends on what you consider are the key skills and knowledge required from a language teacher. Let's consider two: Language Proficiency (LP) and Teacher Language Awareness (TLA), the latter being the metalinguistic knowledge about the language and the procedural knowledge about how to present grammar to the learners.

On LP, native teachers (at least with some higher education) usually outperform non-native teachers. You have a guarantee that they will be able to identify pragmatically correct and incorrect utterances, and correct them appropriately, and that their LP will allow them to talk in L2 in the L2 classroom.

On TLA however, studies demonstrated that native teachers have usually a much lower TLA than non-native teachers and a very low level of meta-linguistic knowledge (Chandler, Robinson, & Noyes, 1988; Williamson & Hardman, 1995; Wray, 1993; Andrews, 1999).

I personally consider that the 2 main requirements for a language teacher are a sufficient language proficiency (min. B2 from CEFR, ideally C1) and a procedural knowledge about language learning and teaching. The common problem with native speakers applying as teachers for a language institute is that some of them don't have any training in language teaching. If I had to choose between a native teacher without any formal training in language teaching and a non-native teacher with a B2 level in the language and some formal training in language teaching, I'd definitely go with the latter.

I found Serge's comment comprehensive; however, I would like to add another point in favor of non-native language teachers.

Non-native teachers have gone through the processes of language acquisition and can much more easily relate with their students in the face of probable problems. Therefore, I would also go with a non-native teacher.

Nevertheless, factors such as the context of language teaching and the purpose for which students learn the language may affect our choice.

I have worked in ESL for 30 years and 15 of those years was as a Program Director. My policy over the years was only hire non-native teachers. We had consistent labor problems with natvie speakers. Work permit probelms and these inability to adapt to the instituttion because of no or Little knowledge of Spanish. Finally, we focused on non-native teachers and it is now ans offcisal policy. Also, over time I have come to the conclusión that the native/nonnative issue seems to be something kept alive by monolinguals. I really think it is not relavant to the profession. There are qualified language teachers and non-qualified, the rest is just markeing spin.

Well trained non-native teachers can perform as good as the native teacher. The issue is not whether the native is native or non-native but the individual professionalism and the ability to deliver. Most often, it is even better to employed non-native teacher from  the language school environment. It helps the learners to learn better. 

I don't think that being a native or non-native (what these terms actually mean is not well established) can define teaching ability. Certainly a teacher who has grown up in the same country as he/she is teaching in will have a good understanding of student difficulties when learning English, as well as the teaching style and topics which are likely to stimulate students.  However,  some native teachers live for a long time in one country and acquire this knowledge too.  I think the only way to judge a teacher is on student satisfaction and progress.

Dear Kemal

Read my article relevant to your study, shared on researchgate

good luck with your research 

a.