Do note that the offer only applies to participating properties in the US region and territories only, for region such as the Caribbean/Puerto Rico, I would definitely call to confirm before making the booking.
All stays have to be made before July I think, can't remember the offer dates, because some properties only claim the charges after you check out.
Some bloggers are also saying that AMEX is being a stickler that you have to put $300 per transaction in order to qualify for the rebate.

If you travel for work, it's not hard to redeem $300 worth to hit the $60 rebate, as most properties in the states avg about $100-150 (inc taxes, YES taxes do count because it will code as starwood purchase in AMEX) per night for four points/aloft. If you go for the higher end properties, it can go about $150-200 avg per night. Throwing in incidental charges such as meals/room service, you can definitely hit $300 easily.

I'm not sure if the $300 offer will read the starpoints purchase offer now as a starwood purchase, but I'm giving it a try next month to maximize my points value.
Planning to redeem points for a trip next month.

At the end of the day, if you have 2 cards, my advice is best to keep them until the new redemption charts are announced later this August.
I have 2 (personal and bis) and my spouse has 1. Yes, there will be an annual fee, but we can also get 3 free nights cert after the AF hits, the potential can do for a lot more value that cost more than $100 per night on these certs depending on the new redemption charts that Marriott is announcing.

If you are worried about paying the AF now, call in after your AF hits, your chances of getting a retention offer is fairly high, as I got more points bonus stacked with some ongoing promo.
It's helping me accelerate to another bucket load of starpoints to redeem those 270k marriott travel packages.
You can always cancel the card after the AF hits, and AMEX usually waive the AF when you call in within 30 days after the AF charge.