Some would argue that UA miles are "easier" to spend.

If ticket prices are drastically different, or if one is consistently a few dollars less than the other, then I'd probably say choose based on that and forget about status altogether. If they really are nearly identical all the time, I personally prefer the product and service on DL over UA, but that's a matter of personal preference.

The other thing you could think about is aspirational award travel. UA miles/status works for you if you're ever looking to try other *A carriers like SQ, LH, etc. DL gets you SkyTeam which..well..DL has nice J cabins, KE is decent, and AF/KL/AZ are still trying to catch up.