You may want to wait until you receive a better than normal offer. Since Amex will normally only let you receive the signup bonus once per lifetime, and frequently offers promotional signup offers, it can be worthwhile to postpone signing up, so you don't waste your bonus on a less valuable offer.
You can earn Membership Reward points from many different credit cards. While you can normally only earn each signup bonus once, there are more than a dozen cards with Membership Reward signup bonuses. You can combine multiple sign-ups to earn hundreds of thousands of Membership Reward points.