This isn't a tough choice at all, if you want to work at Delta then do the Delta ATP-CTP. Going to a regional runs the risk of a training failure and even 6 months at a regional, the lost wages and seniority will more than offset the $10k your ATP-CPT cost.

Here's a little perspective, I had to delay my Delta indoc by 2 classes due to my military obligation, I regularly have to fly trips that a guy 1 class in front of me skips because of his seniority over me. I also know 2 people who are 1 seniority number apart, 1 has been a captain and the other an FO for the past 2 years based on that 1 number.