I would go with the 20k miles, assuming the 13,700 will go unused otherwise.

32,500 miles will get you a DL domestic flight with one leg at "low" mileage level and the other at "medium" level. Not the easiest to find, but not impossible either and what you do find for 32.5k miles will probably be selling for more than $200.

There's also SkyMiles Marketplace, where you could probably get >$200 worth of gift cards or hotel reservations for 32.5k miles. I know redemption rates are worse for non-elites on DL.

If for some strange reason you have a DL Amex, pay with miles would let you get $325.00 worth of non-mileage earning flights.

Miles are also better for those who travel infrequently, since they do not expire until 2 years from the time of last activity in your account. It's also trivial to extend mileage expiration (e.g. a small online shopping purchase linked through SkyMiles Shopping extends expiration for another 2 years).

Advantages of the voucher include ability to earn miles with flights purchased (though this probably isn't relevant to you) and flexibility (i.e. not having to spend time searching and waiting for 32.5k award availability, if it ever becomes available).