AFAIK the original eGo design was made by Joye and first were sold through Janty (which is also Joye). Totally Wicked was also one of the first to have these and they were re-branded as Tornados but still made by Joye.
Eastmall was the first place to carry the Rivas. They were advertised before their release and this caused a lot of heated debate with people claiming that they were stealing Janty's design. It even got to a point where the people from Janty were claiming they were going to go after them, but apparently nothing ever came of that. The Rivas were released, and although they didn't carry a warranty and they looked a little cheaper looking, they sold well because they were competitively priced and many people couldn't tell a difference. If you were to ask me a 9 months ago which one you should get, I would have told you the eGo because it was already proven to be a pretty solid device and it came with a warranty.
But now.. I would probably tell you to get a Riva for one reason only. The new Rivas now output 3.7 Volts compared to the eGo and first generation Rivas which were more like 3.2V under load. That means you can get about the same type of vape using 2.0 Ohms atty on a 3.7V Riva (6.8W) as you would using a 1.5 Ohm atty on a 3.2V eGo (6.8W). Both setups produce 6.8 Watts, but the 3.7V/2.0Ω Riva does it with 1.85 Amps and the 3.2V/1.5Ω eGo does it with 2.1 Amps. Less current means less heat, so in theory the Riva should run cooler and last longer than the eGo, assuming the build quality is equal.