Not necessarily. You can earn points which are convertible to miles by using a Membership Rewards or SPG credit card, in exactly the same way as you would earn scrip points with ThankYou or Discover. The difference in earning is that you may also earn some miles from flying or other activities, but the primary difference is in the redemption. If you will be satisfied to fly transatlantic in economy class there is probably not a big difference, but if you value the greater comfort of business class I would opt for a mileage approach. Ignoring bonuses, it takes about $80,000 spending to earn enough miles with the SPG card for one transatlantic business class award ticket. To earn the same ticket with TYP or Discover would require about $150,000 spending.

Don't panic if those spending amounts seem high. The bonuses will reduce the actual requirements substantially, and with a year+ to go you can pool bonuses earned through different cards.

When you do fly, which airline do you use? What rewards do you earn with your present credit cards?