Romo is an excellent QB, but he chews up cap room when he can't maintain his health on the field. His last 2 seasons he spent significant time out, both injured early on. So chances are you spend a lot of money on a QB that probably won't see you through the majority of the season. And even if he did, he doesn't have the team to win a SB with because the defense still sucks the big one.
With Dak, you get a developing QB for cheap. Dallas need to use this time to put a good team on the field. In my mind that's the best chance of winning. Romo's a great player but he can't stay on the field and his cap hit means Dallas (who can't make good personnel decisions when they do have money) just field a woeful defense. If the front office was better in Dallas they could form a good team with Romo there, but they can't. Next year I'd like to see Dallas trot out their ultra cheap, developing and completely serviceable QB with a strong offense and defense. Like what Denver is doing now.