If you use it as a dollars off voucher you will earn miles/MQMs on the flight you take but if used as 25,000 SkyMiles you will not earn miles/MQMs for the flight. Also the $250 will give you far more flexibility than the 25,000 miles as low inventory can be tricky to find.
IMO, the $250 is much more valuable than the miles.
Thanks for the input, do you also think (normally) delta mile may worth less than $1 per 100 miles?
I only fly AA and UA, if any AA or UA voucher $250 can be exchanged for 25K, no doubt that I will do the exchange.
It seems delta mile has at least 1/3 less value than AA or UA