Georgianne,
Dave is right on both points. The Magic will pass Grills about 5:10-5:15. On Saturday night you also get to see the Carnival Pride leave.
I think we got to Grills about 4:30 and requested a table outside. We had to wait about 10-15 minutes but they had a band set up in the outdoor bar area and we enjoyed that while waiting. We got a table right inside the big open windows and right near the door. We were nearing the end of dinner when the Pride went out and my DH went out and took pictures.
We were finished with dinner when the Magic went out and we both went outside and waived with the crowd. The cool thing about that day was that there was a fishing tournament going on so there were lots of people in the area. Grills was packed! Also it was the first Western Cruise of the Magic that was going by. As the Magic passed Grills the captain blew the "When you wish upon a star" horn. The place really went wild there as well.
I've never been to Jetty park, but there are lots of people out there as well. Since there is lots of space you aren't as packed in.
Don't think you can go wrong with either.