They're certainly NOT completely different. There are a good number of design elements shared between the two models, but they ARE different enough to argue that they were independently designed...
...I'll note that I'd be skeptical of that argument regardless. I'm going to try and compare the release date of this model (if I can find it) in relation to when Slayer's sketch was released as well as Brand's model.
*Edit:
Here's a page showing off their WIPs - If you scroll down, the date is June 28 2010...
http://www.sovereignreplicas.com/The glaring similarities are glaring. Perhaps someone should send an email?...
*Edit #2:
Here's the source of the E-Wing model, actually:
http://fmmodelco.com/