I've had both, currently have the 50 watt EVH.
The Rivera is a complete beast. Its very, very loud and the volume taper goes from 0 to paint peeling with the smallest of adjustments. I found the funny thing about that amp is that it delivers a ton of gain, and then it has a gain boost that is completely unnecessary. In any event, it was a cool amp and I sort of miss it. The EVH is a great amp all-around. I'm sure from a quality point of view, the Rivera is a few notches up the scale, but then again I've had quite a few EVHs and none have failed.
If you can get them both do it.