This past MLK week we did 2 days Universal plus 2 days Disney. We drove on a Saturday, arriving in the evening, so aside from squeezing in a couple of hours on arrival day, our main "in-park" days were a Sunday, a Monday which was the actual MLK holiday, and a Tuesday and Wednesday. We decided to put Universal first even though weekends and holidays are more crowded, with the reasoning that the (
Unlimited) Express Pass is a more powerful weapon than
Genie+ and ILL$, and I think we made the right choice. My subjective impression is that we saw lower crowds at Disney as the week wore on, but that the EPs made up for higher traffic in Uni.
Note, if you're talking about purchasing EP, there are two tiers: one that lets you ride each ride only once and one that gives unlimited everything. If your trip is short, there might be little difference, since you might not have a lot of time for re-rides, and those you want you could do by waiting until park close. But you might consider booking a room at a premier resort for one night so you get two days of unlimited EP.
We went for Hard Rock hotel for two nights, then moved out to Floridays to enjoy having two bedrooms during the Disney days.