Thiel cs 2.3 or B&W N803,804?

I wuld not limit your choice to these two brands, there are plenty more to seek out. lus you need to review your component balance and cable matching, as well as power cabling and mains purification.
Look at the Von Schweikert VR5 (www.vonschweikert.com).

Forget about both of them and audition AUDIO PHYSIC TEMPOS

I am not familiar with Thiel, but I do own a set of BW N804's. I know they need the right electronics. I have a Classe 300 Watt amp that does it justice. However, I have heard them sound better with better electronics. But if I had my choice (and cash), I would spend the extra "g" and get the N803's. For the price they give a good return on the investment. more depth and just a more "musical" speaker. But again, the right componenets make all the difference.
JB

Personally, I like the N804s best. There's really not much of a difference between the 803 and 804s (7" woofer vs. 6.5" woofer and larger cabinet), but certainly not $1,500 worth. If you mate the 804s with excellent electronics (in my case, Musical Fidelity gear), the 804s sing. Haven't heard the Thiels.