Well, I've had a pour of the 6yo, and I'm now in the midst of a glass of the 18. There's a definite family resemblance, but the 18yo shows its extra aging; the 6yo has a rough edge to it that has been polished out in the 18yo version.
I'll have to try it again after the new 6yo bottle has been open for a couple of weeks - my current 18yo bottle has a few weeks on it (though mostly full).
In any case, it is still quite a worthy whiskey, and I'll be getting another bottle of the 6yo for reserve, just as I did with the 18yo.
Although I usually don't try to "score" drinks anymore, I'd give the Saz 18 five stars out of five, and the Jr. would get four-and-a-half.
Next time I get in the mood to mix up some Sazerac cocktails, I'll have to try one of each again. The younger version might actually work better in a cocktail than its older brother, but we'll see.