I'm a MM+ on United based in SEA and they treated me fine until 2013 and 2014. While I was consistently 1K, I wasn't never getting my upgrades approved on International flights and at the end of the year, I'd be left with most still intact. Then United reduced the number of SEA flights which made many of my trips 2 segment trips which isn't great.
I moved all my business to Delta in 2015 and qualified for Diamond in Q3.
While flying isn't dramatically different in any of the US based carriers, there are some slight differences. YMMV.
It's marginally more difficult to achieve Diamond which help in my case since I fly a lot. I think it will help me as I'll have fewer peer competitors for upgrades although going in and out of ATL has never given me an upgrade.
In general, my impression is that Delta has happier employees. Some have been an example to us all of enthusiasm and great customer attitude.
Upgrades seem to clear at a slightly higher rate domestically although I don't have enough data point internationally.
Gate boarding is chaos under Delta. United was quite structured and I miss it. Perhaps they need a few cranky gate agents to get the crowd under control.
On-board movies are changed less frequently but there is a greater selection on Delta.
United did a nice job with international travel through the star alliance but Delta doesn't have the same lounge presence/support internationally. I've been stuck at a number of airports without a lounge including HND, UIO, FCO. The LHR situation is confused across two terminals and the arrivals lounges are hard to find. They used to offer the Virgin departure lounge in LHR which was fantastic.
These lounge issues really only impact you as a traveler when you are a level below Diamond because when you reach that level, you are granted membership to their Sky Club which has alliances with partner lounges.
Note the Sky Club lounge is tighter on bringing friends and family so it isn't quite as helpful when you are traveling in a group. One of the optional Diamond selections is upgrading the Sky Club membership to group but then it only allows 2 others to join me in a visit.
The food and drink options are somewhat better. They have a wicked spicy Thai soup that's fantastic.
Free drinks in the Delta version of economy plus. I was given a small number of free drink coupons at the beginning of the year but still haven't had a chance to use them.
For me it was a good move.
Safe travels.