No one besides Bond fans want and know who Bassey is but Adele would put young people who wouldn't normally go to see a Bond film in the seats.
Then why was she one of the biggest and most popular acts at the Queen's Jubilee just a couple of months back? Everyone knows who Shirley Bassey is, she's huge. Everyone knows the songs Goldfinger and Diamonds Are Forever because they are burned into pop culture and still played all the time on TV, radio and other media. Even alot of people who may not know who she is would hear the news that the singer of Goldfinger, DAF and Moonraker is doing another, anniversary Bond song. They'd think, oh right, they are the big classic Bond songs, interesting. They'd then have found out who she was and be intrigued at least. The news of a return of Bassey for the new song would put plenty of other people in the seats also. It would generally be a big bit of news and publicity overall. Maybe an Adele song in the titles could statistically be percieved as likely to attract even more extra people than that, but that just gets into odd territory.
This film is going to be massively successful no-matter what the song is. So go for the best choice at this stage, not just a number-crunching choice. Put the cherry on the top and dress it beautifully, rather than some cheap sprinkles for kids. Give the film that extra substance and timelessness rather than just whats hot right now to earn a few extra dollars from youngsters. Have a bit more pride in the James Bond franchise than that.
Anyway, you may be right about the marketting point of view. I just would personally rather something great than something bland to appeal to the masses, especially considering all the care and attention to detail they have clearly been crafting into this film for the big 50.