Just bear in mind that this voucher might not be possible to redeem online, at least what happened to me last time. I had to call and book over the phone which ended up in +25€ ticketing fee which they didn't want to refund. And of course 30min spent on the line with TK CS.

That cash refund cheque is relatively easy to cash, just e-mail local THY office and they send you a form to fill in with your bank details, send back with passport copy and money should be on the account within days.