The reason only the SMP (WDTVLive Streaming Media Player) & the HUB are the only one's listed is because the Live and Plus have been discontinued.
The differences between the Live & Plus and the SMP (the Plus being the closest to the SMP) are that the SMP has:
Built in WiFi
Better chipset (processor)
More Internet Streaming Services
Capable of using custom themes
Still fully supported
The Live and Plus' that you find on Amazon are most likely units that someone has left in stock and your going to end up paying as much or more for what is now a discontinued device. I think the Plus is going for about $169 or so (which is about the same price as the HUB) on Amazon, you can pick up a SMP at Best Buy for about $90.
And the HUB and SMP are basically the same, other than the SMP has built in WiFi and slightly better processor and the HUB has a 1TB HDD and Twonky.