You will be able to get reservations on the day of your visit *if* there are still unsold seats still available for the dinner. Its a gamble as to whether that date will still have availability or not.
Do keep in mind, though, that the best seats (ie, located directly along the glass) are assigned on a first-booked basis. So the earlier you book, the better your seats are going to be. There are no "bad" seats, per se, as tables are arranged along the glass and only two tables deep, but you can almost guarantee yourself the second tier seating if you wait until the day of to commit to purchase your tickets to the meal.
We thought was a good value for the money. The food was excellent and the whale presentation was very interesting and informative.
My Christmas trip report (see link in my signature) includes a write up and photos on Dine With Shamu. That portion of the report starts on page 6 at post 82.
I have never dined at Sharks Underwater Grill, so I can't comment on which is better. I have done the all-day dining, and would highly recommend it if you could not get a DWS reservation.