Well, to be honest, I haven't even seen an SR600 in my neck of the woods (might be rare to find) but from what it looks like, the SR800 with the active pickups will give you more range, more control over your tone. They come with a 3 band EQ, the "newer" SR800 models with the stacked knobs have a mid-sweep as well. I've owned a couple of SR800 basses, they're terrific if you can find them inexpensively.
Just pulled up some info on the MIJ SR600 from 1999, from what I can see, 3 knobs - 1Vol, 1Balancer, M.Tone control. So it may not even be an active bass at all, I could be wrong but I don't see a battery cavity on the back.
Maybe get the SR500 if you want the humbuckers and a more recent model, get a SR800 if you want a higher quality but older bass with PJ pickups.