Like Driftless, I suggest both!
I understand the annual fee is a bit to swallow, but look at the perks you'll have with both. They compliment eachother very well and both offer very different yet rewarding perks.
The best thing about AMEX Plat (IMO):
1. Uber Credit
2. Airline Credit
3. Gold status at SPG/Marriott as well as Hilton.
The best thing about CSR (IMO)
1.$300 travel credit (automatic credit)
2. 3x points on almost everything you buy when traveling... their travel category is HUGE!
3. Best rewards point value
I travel roughly 20-30 times a year on average, so i'd like to say i have a decent understanding on what's important when traveling. When traveling, comfort is king! Having gold status as a minimum with most hotels is a huge plus (AMEX). Couple that with maximizing your points with the CSR and you'll feel great about spending your hard earned cash!! 
Negatives:
i've found that the AMEX Membership Rewards don't have nearly the purchase power that CSR's Ultimate Rewards have. Also, Amex doesn't give you the ability to transfer points to United!!!!
A turn-off with CSR... you're going to be very hard pressed to find one!