Diamond will probably make a bigger difference and provide tangible value for domestic flights, particularly if you plan to travel high traffic biz routes or those routes with Delta One. Beyond that, there really isn't that much of a difference in between Platinum and Diamond if you are traveling internationally as both offer C+ upgrades and lounge access, unless I am missing something. Ultimately, it comes down to what type of award and destination you are planning to go to as the sweet spot award deals vary by alliance.