Both Native English-Speaking Teachers (NESTs) and Non-Native English-Speaking Teachers (NNESTs) may have advantages as writing instructors, yet little is known about how they actually influence writing in EFL contexts like South Korea. To address this issue, 76 high proficiency Korean EFL university students from the International Corpus Network of Asian Learners of English (ICNALE) were separated into a group that received extensive native English speaker instruction (n = 57) and a group that did not (n = 19) using a self-survey. Analysis of essay content revealed that words used by Korean EFL writers with more NEST instruction are similar in difficulty and variety to native English writers. Concerning style, Korean EFL learners with more NEST instruction used stances to evaluate, validate, and state personal opinions, while learners with less NEST instruction used speech formulas and more unbiased logical arguments. Regarding grammar, Korean EFL learners with extensive NEST instruction used more sophisticated vocabulary, word forms, and verb tenses to create a novel argument using personal experience, whereas Korean EFL learners with low NEST instruction tended to use formulaic and logical arguments with more accuracy. Overall, results suggest that NEST instruction in Korea promotes more creativity and sophistication in composition, while NNEST instruction encourages more accurate use of language. In accordance with the findings, curricula or teacher training may be developed to ensure that each type of instruction shares the strengths of its counterpart.