I had a very similar situation many years ago. I was at United on the line on probation. I also wanted to be at Delta (although at the time, united was the 900 lb gorilla, best contract, fastest growth, fastest progression, etc).

I interviewed at DAL, I remember one of the interview questions was "have you ever been interviewed and not hired by an airline?" My answer was "no." Then the guy says "so you haven't interviewed at United?" And my response was "I work at United." Next question "how soon can you be in class at Delta?"

I took a leave of absence from United while I was in new hire class with dal and for most of probation. Finally, I resigned from united. I haven't ever looked back. It is a good problem to have.

From a career standpoint, you won't make a mistake from one to the other. However, for me and maybe me only, the cultural change made all the difference, but that is hard to quantify.

Definitely stay with United until you are in class at Delta. No brainer.