It does not matter much, given what you have said.

You will not get credit for UA status, or the UA premier bonus if you take the miles on UA. So if you take the miles on UA you get 2K miles and no credit towards status. If you take the miles on Delta you get 2K miles and credit towards elite status (which you say does not help you).

If you booked online on Delta you may get a web booking bonus if you take the miles on Delta.

Other factors to consider are when was your last activity on both airlines (3 year expiration after last activity) and what is your current mileage level on both airlines (are you saving for a big award?).