Delay discounting (DD) assessments offer a wide variety of procedures to suit specific clinical and research needs. This study compared the reliability and validity of two DD tasks: (a) an adjusting amounts task presented on a computer (AAC) and (b) the 21-item Monetary Choice Task, which was administered online (MCT). Participants were 1,573 Spanish young-adults reporting past-month substance use. Measures included quantity and severity of drug use (i.e., cigarette smoking, cannabis, alcohol) and two DD assessments (i.e., AAC, MCT). Reliability was assessed using both the classical test and item response theory. Correlations and linear regressions examined the validity of both DD tasks in relation to substance use. The MCT showed higher internal consistency than the AAC (α = .941 vs. α = .748). AAC precision was adequate for moderate levels of discounting (θ values between -2 and +2), but the MCT showed superior reliability at low, moderate, and high levels of discounting (θ values between -1 and +1.5). Both DD tasks showed more significant correlations for alcohol-related measures (|rs| ranged between .053 and .093) compared to cigarettes and cannabis. The incremental validity of DD tasks in relation to nicotine dependence (AUClogd: β = -.664, 95% CI [-1.256, -.071]) and alcohol problems (AUClogd: β = -3.098, 95% CI [-5.209, -.988]) was only supported for the AAC. The MCT was more reliable than the AAC for measuring impulsive choice in young adult substance users. Nevertheless, the AAC may serve as a valid marker of nicotine dependence and alcohol problems. (PsycInfo Database Record (c) 2023 APA, all rights reserved).