It really depends on the team as the quality of clubs in Texas vary from year to year. Recently the Solar 94 (loaded with talent such as Taylor Smith and Heather Magee) and Dallas Texans 94 clubs have both been ranked in the nation's Top 5 clubs. Lonestars 93 (Satara Murray, Bianca Brinson) has a very good club as well. In the younger ranks, Sting 95 is currently ranked No.1 and defending national champions in their age group, while Dallas Texans 95 also is ranked in the Top 10. But you are right that the North Texas area clubs are usually superior to any other part of Texas when it comes to club teams. Lonestars could be put in that group as well, while Challenge in Houston has been steadily improving their clubs recently.
But as far as comparing nationally to California, there is really no comparison. California is loaded with most of the top clubs in the nation, with the Southern Cal area containing most of the nation's elite level players year in and year out. Teams like Slammers, San Diego Surf, So Cal Blues, Real So Cal, Legends, Arsenal, Pleasanton Rage, etc. are all superior programs to most of the clubs in Texas. There's a reason that Coach G has all but abandoned Texas kids for the more fertile Southern Cal recruiting grounds. If you look at the national championship caliber rosters in college, most of the best players from those rosters are from California. I think Coach G realizes this and has adjusted his recruiting tactics accordingly. Now, there's always going to be some really good prospects in Texas at the top end (Top 10-15 or so), but they would probably only rank around 25-40 if they lived in California).
So, while there are always pockets of talented clubs around the US, California is where most of the best clubs are because of the sheer volume of elite talent in such a small area. Texas isn't bad, but we still have a ways to go to match up with that level of talent.