iOS is more polished and has more room for actual design. Android is a drastic improvement on what was running on smartphones before, but the built-in UI doesn't have the same fit and finish as the iPhone UI.
For example, look at the iPhone status bar (the bar at the top of the screen that shows the battery, signal, and time). Now look at the default android status bar. The iPhone one just looks better because all of the icons are the same height. The android icons' goofy roundedness only almost fits with the other standard UI elements. Designers/Developers on a polished platform will make more polished apps. Designers/Developers on a rougher platform will tend to make rougher apps. Note that there are exceptions, but this represents the overall trend.
For example, look at the iPhone status bar (the bar at the top of the screen that shows the battery, signal, and time). Now look at the default android status bar. The iPhone one just looks better because all of the icons are the same height. The android icons' goofy roundedness only almost fits with the other standard UI elements. Designers/Developers on a polished platform will make more polished apps. Designers/Developers on a rougher platform will tend to make rougher apps. Note that there are exceptions, but this represents the overall trend.