We were just at Universal on Saturday and walked right on most rides, especially if you head off towards the Simpsons/MIB instead of towards Shrek/Mummy first thing. I would say you could safely go either day and still have lighter crowds.
At IOA, granted this was before HP, Sunday was less busy than Monday over there. I'm not sure if it's still that way. Monday this year was super crowded.
I think Universal just has much lighter crowds this year. We started off going towards the right instead of straight like I mentioned and were able to walk right on Simpsons, MIB, Jaws, went back to Terminator, and then to the Mummy all without waiting. I noticed Jaws, MIB, and Simpsons all had 20+ minute waits later in the day, so I was glad that we got them all over with early without having to wait. If it was me, I'd go Sunday. I think a lot of people are sleeping in or hungover on Sunday mornings, lol.