The SBVT and S300 are pretty different. The number one issue that folks have with the 300 is the gain structure - not a lot of room before you start getting OD. I personally don't have that issue, but I can tell you that the SBVT has a lot more gain before you start hitting OD and it's also a softer, more gradual approach to OD. In other words, the 300 OD is more abrupt. That being said, the 300 will get hairier, much more OD and distortion than the SBVT, so there is a trade off.
The SBVT doesn't have a "bright" footswitch, which is something that I love on the 300. I can dial my gain up to the point just below OD on the 300 and then kick into good OD with the "bright" on. The SBVT OD is super, super smooth tho.
I think the SBVT sounds really good on fretless, I find that I have to tweak the 300 a little more.
I think the best way to sum up the differences between the two is like this (and I hope this makes sense) - the SBVT is a preamp based on the SVT while the Super 300 is more like an SVT in a box. The SBVT is more transparent - the good qualities and the flaws on the bass come thru, while the SVT takes on more characteristics of playing thru an SVT with an 8*10. The SBVT is definitely smoother - I chalk that up to the tubes - but the S300 is dirtier and grungier, and it also has the bright footswitch.
All in all they are both winners. I've been looking for something like the SBVT for a long, long time - a preamp that imparts some character but doesn't overwhelm the sound, and the 8 position mid switch is fantastic, esp for fretless. If I could have one change I would make the mid freq variable rather than 8 position, but this is a small gripe. I vastly prefer the SBVT over a lot of pedals with true parametric.
The S300 fulfills a different purpose, and it's purpose it to ROCK. Thick, grindy, juicy. It reminds me of playing through a big old tube amp, and the OD is top notch.