Probably the U.S. would've been worse off, perhaps fighting a civil war over the issue of slavery. Given the relatively non-contentious death of the institution over the course of the 1890s and 1900s, it's often forgotten how serious of a matter it was to the United States over the course of much of the 19th Century. The extension of the Missouri Line to the Pacific, leading to the entry of the slave states of Colorado and New Mexico, followed by the slave states of most of Mexico, did much to ease the sectional tensions as it provided an outlet for the South to remain competitive in the Senate versus the North while removing the fear the Free Soilers of the North had about slavery possibly extending into the territories they wished to settled in the North. Perhaps with retaining the Northern tier of Mexico issues can still be avoided, given that was where most of Slavery was extended into.
Economically, I still see the U.S. being the top power like it is today. The mineral resources and tourist spots of the Mexican states, while helpful, are not fundamental to the success of the United States. Given that, perhaps we'd have a GDP of $20 Trillion today instead of the $25 Trillion like we do now. Socially, I'd expect a lot less Spanish loan words in American English, as well as Spanish being better preserved in the rump Mexico than it is today. Mexican cuisine, fashion and music would probably be a lot less influential. Race relations would also be much different, as the adoption of the Latin American concept of Blanqueamiento would be less likely to gain traction like it did IOTL. Perhaps the One Drop Rule would've had staying power?