Wildstar is a subscription game, GW2 is not. That's one major difference.
Although I thought Wildstar had some funny bits for the one weekend I was allowed into beta, but it's combat seemed less interactive than GW2. Which can probably be blamed on it being beta and not perfected yet...we'll see. Hopefully they changed to non-subbed.
Personally I see Archeage being the real threat to GW2.