For what day, Friday and Saturday are generally busy no matter what. I'd say Friday, but I don't think there's a significant difference between the two.
However, Express is highly recommended for either day. I've seen a good chunk of people last year who regretted not getting it. You'll have a more relaxing time with it, and when you combine it with the Stay and Scream (which you'll need a daytime ticket for), that's the best way to hit Stranger Things.