I've only stayed at the Marriot once between the two. I would definitely choose that place again if lucky.
My friends and I would always buy alot of stuff so not having to take a shuttle and or walk long distances was a huge plus. The only downside to being that close was that we bought more than we should've aha
Besides being next to the convention center, you're closer to other places in Gaslamp (love going to Ralph's). Plus, I think there seems to be more Con stuff going in and around the Marriot.