Unless you do not see yourself staying at a Marriott in the next 12 months, the voucher is the better deal because it can be used at any Marriott property (including category 7) with no capacity restrictions. Basically, if a room is for sale, you can reserve it using that certificate. Those certificates are usually given out to elite members when they are walked from FS properties (and all properties for platinums) because the hotel is oversold.