The present paper investigates how speakers of varieties of English select between syntactic alternatives across registers by exploring variation in the constraints determining the choice between overt and omitted pronominal subjects in BrE, IndE, and SgE. To this purpose, relevant observations of the variants were annotated for several language-internal constraints, as well as for mode of production, level of formality, and variety, and subsequently submitted to variationist modelling. Results indicate that the most reliable constraints are also the most stable ones across varieties, and that mode and level of formality outperform variety as predictors of pronoun omission. Therefore, the present study sheds light on how the effects of language-internal constraints are modulated by external factors, particularly register and geography.