Whichever one has the best weather forecast and best fits into your plans. Activities are pretty much the same every day.
Also they've been selling-out quite often this year so it may already be too late to get tickets for one or the other. I'd recommend making a decision quickly.
Went tonight. So many people. And the cookies and cocoa lay out in the 7-9 time slot is not conducive to holding a spot for the parade. All in, not that pleased with the value you get for this party any longer.