Might depend a bit based on my favorite destinations, but just considering global reach, award availability, and contribution to lifetime elite levels, I'd take AA miles over UA.

The lifetime elite opportunities are the tiebreaker for me. UA seems to have slightly better global reach and AA has slightly better availability across the variety of awards I've sought. If you gave me 300k, I'd put them in AA and be that much closer to Lifetime Platinum.

AA also has the HHonors conversion, which is about the only air-miles-to-something-else conversion I've seen that makes any sense whatsoever. (I consider it a basic push in raw value, but a great benefit to be able to diversify your travel awards if you happen to earn tons of AA miles.)

The other four "majors" are worth significantly less than either UA or AA, IMHO. Maybe that changes at the top-tier...I don't know. But any of the majors should treat you well if you hit top-tier.