Personally I'd go for the Spector - the Epiphone Thunderbird is just nasty. I've had a Gibson Thunderbird and an Epi and to be honest, apart from the shape, the two basses couldn't be more different. Different woods, different construction, different sounding pickups, different neck measurements, different weight etc.
You can't go wrong with a Spector although having said that Yamaha do make nice basses.