The new line seems geared towards women's fiction initially, but Bantam Dell publishes a fair number of historicals, so I wonder if we'll be seeing some of them here too.
Simultaneous publication in both formats is common in the UK, but not in the US.
Given the choice, and given identical content, would you purchase a $13.99(ish) trade paperback over a $7.99 mass market one? Remember, also, that Amazon discounts most trade paperbacks to $10-something, while mass markets usually get no discount at all.
I admit I'd probably pony up the extra $2ish for the trade size...