Here is my 2¢, pick an airline with CPP if you are just now getting into the 121 world. First, I know it’s not a true flow but you know whether you are accepted with in the first year. If United says no, jump over to a AA regional and wait the 8 years for the flow.(or go to a LCC or ACMI) The CPP programs are also much faster with most sending people in the 5 year mark consistently. Look at long term planning, if you make it to United in about 5 years you will be right at the beginning of their huge retirement wave. If you join an AA regional now and flow in 7-10 years you will be at the very back of their retirement wave. This difference in seniority at the major might be significant enough where if you go to AA you might never get the chance to upgrade, and if you do it will be in the last few years of your career. If you go to United right at the beginning of the hiring wave you will have a huge boost in seniority every year which will not only speed up the upgrade but will also help move into the widebody fleet of you so choose. Lastly is the other programs like DGI, I would steer clear of these as you have to wait years to find out your status of progression. At least with a CPP you know fast and can make adjustments if needed.