you'd get the benefit booking through ThankYou.com because such reservations are pre-paid with the benefit amount applied at time of booking.
you'd get the benefit booking through Aspire if you book a non-refundable rate now, because the benefit would be paid within a few weeks.
i can't imagine you'd get the benefit booking through Aspire on a flexible rate because Citi has to apply the benefit as a statement credit and that would be suspicious on a closed account.
if staying at a Marriott and not wanting a non-refundable rate, a sneaky option would be to have Aspire book a non-refundable rate and then self-service use Marriott's generous 24-hour free cancellation policy to change to a flexible rate.