This study investigates the extent to which (a) Spanish heritage (HL) and second language (L2) writers' linguistic complexity differs across register contexts and (b) Spanish proficiency and writing motivational beliefs differentially affect HL and L2 writers' performance. Participants were 58 HL and 54 L2 Spanish learners who completed two persuasive writing tasks - the Email to Friend and Letter to Dean tasks - designed to be topically similar while eliciting different registers. Proficiency measures included an elicited imitation task (EIT) and a cloze test. Mixed-effects models indicated that both HL and L2 writers evidenced greater lexico-syntactic complexity in the Letter to Dean task; nonetheless, HL writers demonstrated more robust cross-register distinctions in syntactic complexity. The EIT and cloze test positively predicted syntactic and lexical complexity, respectively, although differential patterns were also observed by group. Intrinsic/interest and cognitive/linguistic value beliefs about Spanish writing emerged as positive and negative predictors of linguistic complexity, respectively.