That's such a tough one. SFFT has the best B&M in Texas, Superman Krypton coaster. It has better theming, and is a BEAUTIFUL park. The rock quarry is truly one of a kind. SFOT is more paths to rides. The Texas Giant, IMO, is the greatest ride in texas. However, I won't lie to you, it IS rough. What a ride, though! Mr. Freeze is great (slightly better than poltergeist), and so is shockwave. Judge Roy Scream is okay. I would say you're in a tough situation. SFFT is more fun because of an awesome german restaurant, better theming, better non-coaster rides (especially their log flume) and some great rides. Rattler is okay, but nothing compared to Texas Giant.
Astroworld is okay, but nothing compared to these two. Greezed lightnin' is like any other shuttle loop, and texas cyclone is pretty amazing. Those are the only two there worth mentioning(although texas tornado, which was closed, looks great too)If all you care about is coasters, go for this: For good wood, go to SFOT, for good steel, go to SFFT. Personally, if I was given the chance to visit just one again, I would be equally happy at either sfft or sfot.