How many total miles do you typically fly?

Delta Pros:
- Rollover (assuming DL continues this, you could have Gold status (50k) after two years of 38k flying)
- Likely better upgrade opportunities out of ORD than UA as it is not a DL hub
- IMO a better in-flight domestic experience than UA

United Pros:
- More direct flights from ORD
- Better irregular ops support from ORD (more options)
- More Economy Plus seats than Delta Economy Comfort (more likely you can get them as a Silver (25k) )
- Much easier to use miles (esp ex-ORD) and a better value. If you don't earn many miles (and Silver status does not), this may be a big deal

Neither airline offers free standby to 25k level, but both do for 50k level.

Both airlines offer credit cards that give you priority boarding and free bags, so that may be another option.