I own a SCSD, a SI, and a SII.
They all sound great, they really, really do. Its really hard for me to say that one sounds better than the other.
The series basses do give what to my ears sound like cleaner, hotter, more hifi type of sound than the SD does, and you also have more variety in terms of tones thanks to the series electronics, which doesnt really mean anything to me because I pretty much try and dial in the same tone no matter what bass I am playing.
The series basses are also much, much more sensitive to sloppy technique, the SD is much more forgiving.
On room recordings where I have played both a series bass and SD bass, there is a tonal difference. When I use my demeter/sf2/qsc rig I can almost (but not quite) make that difference go away.
IMO, you are not missing out on great deal by not having series electronics, but there is a difference, and once you have a series bass you will notice it.
Electronics aside, another thing to consider is the woods. The type of wood makes a huge tonal difference. My SI is maple, SII and SD are both cocobolo and there is a huge difference in sound between the maple and coco-bolo basses...
I think if you were to put things in terms of automobiles, the SD is a porsce or a ferrari, while a SI or a SII is a Lambroghini.
If you have the cash and can part with it easily, try and find a used series bass and discover these differences on your own. If you find that you dont need what it has to offer you can easily flip it with probably no monetary loss.
Lastly, if I could only keep one of my bass it would be the SD.
I would miss what the series basses have to offer, but I am finding more and more that the SD really fits all my needs.