Gee, your post seems to have quotes that look familiar

Seriously, the UA seat (business) is very good. It is better than the AA seat. The F seat is even better. I've never been on DL, but I'd expect the UA seat to be better. The only better product from the east would require you to go up to YYZ (AC) which isn't worth the trouble, because not only is it out of the way, but would require a SCL stop.

DL is now partners with AR - do NOT fly AR under any circumstances, unless you are flying within Argentina and it is your only available airline. Repeat. do NOT fly AR under any circumstances, unless you are flying within Argentina and it is your only available airline.

Given your options (i.e., you don't have any miles you can use on LA), I'd fly UA via IAD. If it is later in the year (April +) you have to trek to EWR for UA, so then DL becomes a better option for flight timing. However, EWR/EZE gives you a long time to nap on a flat seat.