Well looking at your programs, UA, DL, and WN - Ultimate Rewards transfers to both UA and WN, amex mr does not.
Honestly if you can just get 6,000 points on the Sapphire Preferred and transfer those to WN, your annual fee is paid for (6k points = $100 airfare).
If you use the AMEX Plat as your everyday card, the points really won't add up to as much as the CSP.