Is this going to be concrete or liner? If concrete, go with the vertical sides and flat bottom. It is easier to pour. 24" to 30" seems a little deep for a fountain (Need to consider the liability factor). 25 foot diameter is going to give you slightly under 500 sq/ft surface area. At only 1 foot water depth you will have 3675 gallons. A lot of water!!
What type of Fountain configuration are you planning.....formal 2 or 3 tier, stone bubblers, free form, etc.? I
As usual, need more info.