The problem I have about the Viper is that it is designed to be similar to an SG. I played one at GC once and everything about it struck me as almost but not quite an SG. To me it's an attempt to get as close as possible to am SG without attracting negative "attention" from Gibson. It would be cool if they changed it up a bit though, maybe go for a nontraditional color and made the pickups wood mount. On the plus side however, they did switch to a master volume/ master tone layout, which I prefer, and they made it a 24 fretter, which you might as well do on an SG since the neck pickup is in a weird place anyway. The side jack is also nice. I never liked how they put the input on the face of the SG. The LTD is also a fine piece of workmanship for the money and the neck seems to be more secure to the body than the G400.
I'd say if you've never spent serious time with an SG or don't think the minor changes from the Gibson design will bother you, go for the Viper. If you care for traditional looks and mojo, go for the Epi. They are both decent guitars and neither is a bad choice.