Depends what you want! They're very different, in appearance, habits, habitat, needs, feeding, level of care - you name it!
Why do want one? I had a green carpet for a little while that was really cool to watch, but others haven't enjoyed the predatory nature they display (fish catchers).
BTAs are also cool if you find an interesting colour, as you have. They are less demanding as well, and don't require a DSB for them to settle in.
And then of course you have to judge the overall health of each animal, and make your best choice on which will survive, and go on to long life in your care. This is hard if you don't have some degree of experience with them to start with.
But a Blue Carpet? Tough to pass THAT up... What kind of lighting/setup/substrate/filtration do you have?
