1) take the delta class date. Passing up a major class date is prima facie evidence of a mental deficiency incompatible with a First Class Medical
2) For context, a historical 27yo choosing an airline who retired today would have done so in 1986. At your proposed choices of Delta and United you could have ended up being a DC8 FO. TWA had retired their 707's two years prior ... so you would have missed that opportunity. The things that will make United vs Delta a better choice in 2050 and beyond are unknowable. Maybe Delta holds out 8 years longer for single pilot ops. Maybe United actually gets carbon neutral SST's. They're both great choices.