There is no difference between Laszlo and OpenLaszlo. The name of the company is Laszlo Systems and when their Laszlo server became open source, it was renamed to OpenLaszlo (which I think is a bit of a mouthful).
RIA development is a huge new area of development, so it doesn't really make sense to ask which language or system is better. Its like asking what is better Ruby or Python, where each language has its own group of dedicated developers. So I wouldn't try and pit Laszlo in competition against other systems like Ajax toolkits such as Ext.js. There is no universal solution to every problem and there are undoubtedly many situations where Ext.js is the superior solution and the same for Laszlo.
But Laszlo does include unique programming methodologies, so if you really want to understand RIA development, then I think a working knowledge of Laszlo is mandatory.
Please take a look at the Laszlo Market application that is build over the course of our book and let me know how you like it. You can find it at
http://demo.laszloinaction.com/products/SKU-003 This is a stripped down version, it doesn't include a functional video player, but we will be continually updating this website over the next few weeks.