Besides of Delta has more FC seats, Delta also offers REAL MEAL in F on
RJ while UA offers snack box. More importantly, Delta is trying hard to
get ride of CRJ200 or EMB145 while COdbaUA loves using EMB145/CRJ200
on many routes more than 1000 miles. You may consider which route you
fly mostly and what equipment will you get. RJ F is still F and much better
than any RJ Y. To have upgrade opportunity to be upgraded is better than
no chance to be upgraded no matter it's RJ or mainline.
Delta also has better customer relations. When you have any trouble and
you send an e-mail to DL, they usually respond in 2-3 days as DL gold
while 1K voice replies in about 1 week and PMCO customer WE DONT CARE
usually replies in about 20-40 days even you were a platinum.
As Delta Gold, you can access Skyteam lounges as UA G can access *A lounges when travel internationally.
Although skypesos is almost useless on domestic award, you have good chance to redeem on partners for international travel.
If you can't hit platinum on UA, you won't get any RPU.
Think clearly before you move.