UA SWUs are on W+ which on DL is K+
DL allows K+ use of SWUs for accompanying persons
DL allows M+ use of SWUs internationally
While UA allows W+ internationally and any fare locally up to Hawaii.
As AndyTLe puts it if Intl upgrades are important to you, DL is not for you.
In many respects US should have merged with DL, NW with AA and UA with CO.
Both US and DL are very similar in their rev management ideas.