tldr; skip the diet drinks. Coming from the Framingham Heart Study Offspring cohort of 2888 people over age 48 "the hazard ratios were 2.96 (95% confidence interval, 1.26–6.97) for ischemic stroke and 2.89 (95% confidence interval, 1.18–7.07) for Alzheimer’s disease. Sugar-sweetened beverages were not associated with stroke or dementia."