I've never been to SPI, but I KNOW I would love it. Since I've never been there I'm gonna have to go with a beach I have been to, Galveston.
My family and I went down there on a family vacation and I actually had a blast!
I've been to both Galveston and South Padre Island for two seperate school trips (they were both my Senior year of High School). South Padre is SO much cleaner than Galveston! So, I really love and recommend South Padre Island's beach.