We are of the "begin as you mean to go on" school, but also "put on your own oxygen mask first." So which would be better for you? We go out before the first day of school, which means I get to work later than usual and need the extra time from extended day.
My kids are usually wiped out the first few weeks of school, especially kindergarten, so I would pick them us as soon as possible from extended day. But the first week, they went every day, just to get into the drill and see which of their classmates were in extended day, too.