Although there may be some overlap and Simple IoT may eventually support a number of off the shelf consumer IoT devices, then genesis of the project and intent is really for developing IoT products and the infrastructure required to support them.
Particle.io provides excellent infrastruture to support their devices and solve many of the hard problems such as remote FW update, getting data securely from device to cloud, efficient data bandwidth usage, etc. But they don't provide a way to provide a user facing portal for a product that customers can use to see data and interact with the device.