You can see just the tops of the floats from the party, but can't see the parade well. I would not count the view from the party as "seeing" it, if the parade is important to you. We save it for another night.
We thought the view of Magic, Memories, and You was fine. However, that night was the only time we watched it, so I can't compare it to seeing it from right in front of the castle.
They give you wristbands to wear when you check in for the party, so you can leave for a bit, for instance to visit the restroom. I wouldn't go too long, though.