I don't think they're similar at all. Completely different, except that they're both towers. The Carthay is very classy, historic, fits the theme, and serves its purpose beautifully for Buena Vista Street and means a great deal to the Disney company. Very iconic. It's not as ornate since it's very 1920s Spanish looking, but it has a lot of heart. It's grown on me. The Universal Plaza Tower is super ornate and detailed and beautiful, but it's totally unique and as far as I'm aware, has no historical significance for Universal. However, for USH it will look and be perfect and I think it's gorgeous as well. They're very different. I hope it becomes a popular icon for USH.
If it truly is not based on any existing building, props to Universal Creative for making up such a unique and beautiful Art Deco building!