The present study explores the relative importance of register in learner writing vis-à-vis learner-internal factors such as first-language background. Using multi-dimensional analysis, the study looks at learner and native-speaker student writing from two registers (argumentative essays and research papers), in comparison to published scientific articles. The results show that while certain differences could be noted across first-language background, the main differences were found between the registers, stressing its importance as a moderating variable. Specifically, the research papers and the scientific articles were characterized by topic-focussed, factual descriptions, and the argumentative texts by a more personal style. The results thus highlight the importance of taking register into consideration in learner corpus research studies.