As an "either/or" choice, I would put Torcello ahead of the Accademia - because I prefer outdoor buildings and atmosphere to inside art. And as well as the masterpieces, there's a lot of pretty mediocre stuff in the Accademia too... Others would choose the opposite, according to their choices. But if you get to Burano, you're ¾ of the way to Torcello, and it might be a long time before you come back. :-)
On this trip, I think you're wise limiting your time in Padova. The piazza is nice, not as touristy (IMHO) as someone suggested earlier in this thread, but it doesn't compare with San Marco for architecture, sheer scale or atmosphere.
Good luck with your planning. It's encouraging to see somebody taking the time to think things through and do some decent research.