This study aimed to assess sex differences in predictors for becoming a current exclusive electronic cigarette (e-cigarette) user, current exclusive smoker, or current dual user (concurrent smoking and e-cigarette use). This longitudinal study included 2399 females and 2177 males who had tried neither cigarettes nor e-cigarettes at baseline and attended 57 middle schools in the three largest cities in Mexico. We estimated multinomial logistic models stratified by sex. At follow-up, the prevalence of current exclusive e-cigarette use was 6.4% for males and 5.5% for females; current exclusive smoking was similar among males (3.6%) and females (3.5%); dual use was 2.4% females and 1.8% males. In the adjusted model, current e-cigarette use among females was associated with baseline current drinking (ARR = 1.85; p < 0.05), having a job (ARR = 1.99; p < 0.05), higher technophilia (ARR = 1.27; p < 0.05), and higher positive smoking expectancies (ARR = 1.39; p < 0.05). Among males, only having friends who smoke cigarettes at baseline was a significant predictor of current exclusive e-cigarette use at follow-up (ARR = 1.44; p < 0.05). For both sexes, current exclusive smoking at follow-up was associated with baseline current drinking (male ARR = 2.56; p < 0.05; female ARR = 2.31; p < 0.05) and, among males, only with having a parent who smoked (ARR = 1.64; p < 0.05). For both sexes, dual use at follow-up was associated with baseline current drinking (male ARR = 3.52; p < 0.005; female ARR = 2.77; p < 0.05); among females, with having paid work (ARR = 2.50; p < 0.001); and among males, with parental smoking (ARR = 3.20; p < 0.05). Results suggested both common and different risk factors by sex, suggesting that interventions may need to consider targeting sex differences.