On my recent trip to AC another guy and his friend staying at Banana Beach opted for the 1 day tour of Tikal run from the island. The flight to the mainland was supposed to depart around 7:00 or 7:30am but was delayed because of the weather. This shortened the (already short) amount of time they had to visit the site, and put them there later in the morning... the late morning/afternoon is the hotest time of the day. Also, it's when all the other tour buses/shuttles from Belize and other parts of Guatemala show up, so it's also the busiest time in the park. That one day tour, where they got to spent maybe 5 or 6 hours at the park, cost them almost $400US.
I, on the other hand, opted to go it on my own... There were cheaper ways to do it than even I did, but even with flying to Flores (approx $150US); taking a taxi ($40US) to Tikal; the "hotel" room at the park ($33US); entrance into the park ($10US, I think); Sunrise tour of Tikal ($10); Canopy Tour outside park ($20US); personal tour of Yaxha site (where Survivor was taped - $80US); transportation to Flores ($5US); hotel in Flores on second night ($35US)and transportation from Flores back into Belize at the end of the second day ($20US)..... It cost me just about the same amount of money to spend 2 days there and was a much better deal, I think.
Of course, if timing is the issue and money isn't, the day trip will get you there to see the basics. In my opinion, though... if you can stay there over night and do the sunrise tour... it is soooooooo worth it!