Granted the lead character is rarely in real jeopardy in any TV show, but when you do a prequel series it seems to rob even a little unpredictability regarding the fate of the main character.
A prequel show like Enterprise got around that by featuring a totally new (and up to then) unheard of Enterprise and crew. With any of the Star Treks, since they were taking place 'right now' respectively we could never be as certain that they would make it out alive. For example I didn't know if Picard was going to survive "Best of Both Worlds". Or what about Kirk in "The Doomsday Machine"? The Star Wars prequels both introduced new characters with old ones, but one of the main draws of those films was seeing how it all came together and set up the original trilogy.
With the Cylon War I think you could have something similar to the Star Wars prequel films in that regard, but with the TV show known for some unpredictability and twists, I say why not have the best of both worlds and go the Enterprise route?
Adama's story was well told during BSG, plus we got to see him during the first war in Razor, and to some extent his youth was covered in Caprica. He's a great character, but I would've rather seen new characters, totally fresh and new to the universe. It wouldn't have hemmed in the potential writers for the series as much. And it could open up more avenues for them to explore different aspects of Colonial lives and cultures.
Regarding Saul's feelings about the limited places B&C could go...I think there was a lot of stuff they could do to cover the war and how it affected the colonies. Perhaps we could get some new takes or interpretations of what we 'thought' had occurred during the war. Also there could be new Cylon models, Cylon homeworld, new Colonial ships, and a lot of cool stuff. Also B&C might give us more information about the Cylons and Final Five, while also tying up any loose ends from Caprica. Granted it's probably just filling in the blanks, but it could still be interesting. Especially if you base it around interesting characters and life and death situations. War is a terrible human experience that lends itself to constant exploration.