I use the Arrival+ as my everyday card. It provides an effective 2.2% cash rewards. You get 2 points for every $ spent. You cash in your points for any travel expense on your last bill. Each point is worth $0.01. It is simple. Plus you get 10% of your points back immediately.
Regarding your original question, each time you sign up for a credit card, you will get a temporary 3-5 point hit to your credit score. The hit will last for no more than a year. If you don't plan on purchasing a home in the next year, it shouldn't matter. When evaluating your credit history, the credit report companies look at 2 things, your longest credit history and your average credit history. Keep your longest credit card open and closing a recent one will not be a problem. Amex does offer a no fee credit card utilizing Membership rewards so after you get off active duty and cancel your Amex Platinum, you can still keep any you have. There is no good reason to put everyday spending on the Amex Platinum after you get the signup bonus. There are better cards for everyday spend.
There are many ways to spend the MR points:
You can get gift cards (don't)
Buy certain items on their website(don't)
Get a statement credit (don't)
Use them to pay for trips made through Amex (maybe, this is the no black out dates statement you can reserve through them and either pay or get points to pay at a rate of $0.01/point)
Pay for Amazon purchases (maybe)
Transfer them to airlines to use as miles (the best normally)
I have dove head first into this game and can answer any questions.