There mission behind the attack on Gotham city by both Talia and Bane was to, like Ras-Al-Ghul thought, perish the city as corruption in the city went too far for correction. So they planned to attack with nuclear bomb, so that nobody could escape alive from Gotham. The only problem in front of the success of their plan was the Batman. So to make sure the bomb goes off as planned, they chose not to leave the city. Here the psychology of Suicide Bomber sometimes matches with the attitude of them. They knew from the start if they stay in the city they will die like @tyler pointed out, the blast radius is too big for anybody to escape from it. But they also wanted to made it a success, which Batman could hamper. So they decided to rather stay. So we can conclude that they wanted to stay and die for the reason. She loved him and wanted him to join her for the noble (!) cause.
As far as Talia is concerned, I don't think she wanted to leave the city alone or sacrifice Bane. You see, Talia had the trigger to blast the bomb and she did not try to escape while pressing it. Its because of Gordon's try the bomb did not went off then. She did not merely use Bane for the success of the plan.
I think Bane joined her just because they loved each other and he also wanted to join Talia for the so called "noble" reason (Don't forget he was also the disciple of Ras-al-Ghul). So they knew they were going to die. Bane beat Batman because he was angry on him like any criminal should be. He also wanted to make sure that Batman passes away as early as possible, because there is nothing more stupidity than keep ones enemy alive. After all we cant deny he was an A-1 criminal. That is why before Talia left him she bid the last Good Bye to her protector cum lover Bane.