You could probably get AA Platinum or UA Premier Executive being a CO Platinum. I think that AA & UA are both excellent and are both flagship carriers for two amazing alliances. I think that for entry-level flyers, AA or oneworld is the best. Their Gold/oneworld ruby program offers a ton compared to UA Premier/Star Silver but (being a oneworld Sapphire & Star Gold) think that for higher yield flyers (as it seems you are) United is the better airline and Star Gold is also excellent. I was a Premier Executive once and a 1K once and think that they're both great but I've chosen Lufthansa as my primary Star Alliance program. Bottom Line: United