7500 points gets you a $100 statement credit, which is effectively 2.6% cash back. Not bad as long as you spent enough to regularly hit that 7500 point threshold. If you actually use Travelocity a lot, it's even better (5.2% cash back on those purchases).
But 20,000 points doesn't get you a $400 credit, it gets you one free plane ticket or flight/hotel package UP TO $400, which is a big difference. It's still probably possible to get >3% cash back equivalent value using that, but it'll be a challenge to really hit 4%. And keep in mind the $69 annual fee and the issue of having "stub" points if you don't regularly get to 20k.