It depends when they fought. When Sak was in his prime and beating the Gracies (1998-99), Randy was still pretty green (besides wrestling/dirty boxing) and his submission defense wasn't very good. By around 2002-3, Randy was better and more well-rounded and Sakuraba had started to take serious beatings/damage from larger fighters. He wasn't the same.
Another factor is that unlike the Gracies, Randy could outwrestle Sak and work his top game. Being a catch wrestler, Sak's submissions were more likely to come from dominant positions (not his back), so he may have a hard time with Randy--styles make fights, after all. That said, I'd still pick Sak until around 2000-2001...especially in a 1 hour fight like he had vs. the Gracies. He'd probably eventually get a kimura or guillotine or something. Randy's chances would be much better with 5 minute rounds in a cage though. By 2000-2001, it becomes more of a toss-up. Any later than that I'd start strongly favoring Randy.