I just finished up a dram of the new Sazerac "Jr." and am now in the midst of a pour of the 18yo for comparison. For a young age, the 6yo is superb, but the extra aging of the 18 polishes out the rough edges.
Either version is well worth buying, IMO. They both share that lovely spicy-floral-fruity rye flavor.
Next time I whip up some Sazerac Cocktails (I use a variation of Chuck Taggart's recipe, with the addition of Regan's Orange Bitters as well as the vital Peychaud's), I'm going to have to try ones with each version; the last time I mixed Sazeracs, I used Rittenhouse BIB rye, which worked very well. I have a feeling that the more aggressive character of the Saz 6yo might work better in the cocktail than the smoother, more polished 18yo.