The main difference is in the sole plate. There are a lot more steam holes in the purple one. The yellow one is designed to be flatter, but still deliver just as much steam. They claim that it will give a nicer finish to your fabric by preventing "dimples" from forming where you pressed and there were all the tiny steam holes. Also, the yellow ones (1600 model) have a 1.25" longer cord.
You can click on each model here (
http://www.oliso.com/smartiron/)to look at the pics of the sole plates.