I have a few Spectors and this one (the REX 5 Pro) is by far the larger body style. It's virtually the same physical size body as a Gibson bird except it has a cutout on the lower horn to allow better fret access. The Rex has thicker body wings and is a 35" scale vs 34" on the Gibby. It looks and feels a bit bigger than the Gibson but not by much. When strapped on, the Rex feels a bit heavier but again, not by much.
Both are through neck basses but the construction similarities stop there. The Rex is a maple wing and three ply maple neck bass vs the mahogany construction of the Gibby. The Rex is also an active two band preamp vs the passive Gibby. The woods, pups and preamp all combine to give the Spector a more aggressive tone. More grit and growl and a brighter top end along with a bolder deeper bottom end. It really is a beast and a bit less refined than the Gibson. Actually it's a bit more like a Warwick in sound than a Gibson and because of that I find it better suited to heavy metal, grunge, hard rock and some progressive rock.
I only play finger style with no slap or pop technique and have a tendency to favor the neck pup and play mostly above the neck pickup or more towards the neck vs the bridge. With my style of playing and settings I find the Gibson sets in very nicely in the mix as a solid foundation that can come forward in the mix when called upon. The Rex is just the opposite, it's normally more forward in the mix and can be downright in your face when you dig in. Because I'm more of a laid back bass line kinda guy and not really focused on grabbing the spotlight very often I find the Gibson to be the better overall bass for me. But if you want to stand out and be seen and heard in a hard rock setting the Spector might be a better choice.
Note the Rex 5 pro is now discontinued (the similar looking Spector Legend is a totally different animal) but you can still find the Rex used for around $600 with or without the crazy love it or hate it Holoflash finish. Here's a better shot of the wild finish that constantly changes as the light hits it from different angles...
Again, it's a love it or hate it finish but under stage lights it puts on a real show.