My experience has been that the stock solutions in the Kodak six bath kit last a very long time after being opened - at least six months, probably longer. The mixed solutions last a pretty long time too - weeks.
As for comparing three bath to six bath kits, I have noticed that the Tetnal three bath kit produces a bit more dense transparencies than the Kodak six bath kit. That may be due to the blix. Otherwise, the results seem comparable. There may be some stability issues that make the six bath kit better, however. It is worth noting that the Tetnal kits use a formaldehyde type stabilizer.