I don't have a strut or shock tower brace, but I've heard from those that have all three (strut, shock, and x-brace) that the x-brace made the biggest difference of the three. I definitely noticed a difference in my car after I installed it. Feels considerably more solid.
1998 BMW 328is
1966 Pontiac GTO
2007 Subaru Impreza 2.5i 5-door
View my photos: Caught in the Wild