15 years ago I was a continental fan. Moved to Orlando and decided to move to DL. After 5 years of exclusively flying DL (I fly all leisure on my own dime) I got fed up with the poor customer service, lack of transperancy in upgrades, planes were ok but ground and phone staff was rude. I moved to Continental because of that. I know people with silver or gold in DL who get upgraded all the time but it is hard to compare where and when they get upgraded compared to where and when I get upgraded.
I am not a fan of United. I actually like AF (not CDG) and KL and Schipol. The skyteam (non-DL) lounges are very nice for those of us who do not pay for international first class.
It is a tough decision.
I am more than half way to million miler and can make it in 3-4 years.
As they say: "the devil is in the details" and I will have to see what the CO/UA agreement does for me.

Before you switch to DL, I would spend a few hours reading through all the DL threads. All in all, worse stories than with CO.
good luck