I have 1 of each, the Cognoscenti is a crisper & stiffer hit than the Harris cue in my opinion. I like it better because I love the hit and feel of the G-10 pin. The Blue Grass cue hit's very well also, you can't go wrong with either one IMHO.
I once owned a Cognoscenti, and I liked the hit/playability of it. A friend has a Bluegrass that I've shot with several times and I slightly prefered the Cog. The Cog hits a little stiffer and harder. IMO, you won't go wrong with either one. They are both great cues.
With that said, I think you should also consider a Chudy. He, like Cognoscenti, uses the G-10 pin. Chudy's cues hit softer although they have a stiff shaft taper. I personally prefer my Chudy to the Cog I had and any of the several Bluegrass cues I have played with.
I have played with both as well. I will tell you it's a matter of personal taste and what feels best to you and fits your game. I think a big part of the hit depends on the woods used by both makers. I prefer the Cog over the Bluegrass because the hit seemed a little more stiffer and the way the G-10 pin felt. As far as availability is concerned I see used Bluegrass cues pretty often on Ebay and some other sites. I very rarely have seen any
Cognoscenti cues other than plain janes for sale anywhere. When you can find them, they usually are high end with a high $$$ to go with them.
If you factor availability in with price it's up to you. I think Bluegrass cues
are more affordable and easier to get but I think Cogs are a better investment, especially if you factor in that when Joe Gold starts building cues again he probably will be making more rare and real high end cues
more for collecting.