The release weight is the difference. Special triggers are held to tighter tolerances.
If you want to be over 1 3/4 lb go with the Primary if you want to be under that down to 1 lb go with the special. We offer both on LRO if you cant find them.
I've used several TT, a couple of CE, and various other Shilen, Jewell, Timney 510, etc. My favourites are the TT and the CE. The Shilen is right up there as well, but it's hard to find one with a safety around here. The CE has been very crisp and reliable. The TT's have probably had the best feel and adjustments, but I've had some reliability issues. TT has awesome customer service and were happy to work with me until things were resolved, so I continue to buy their triggers. The Primary feels about the same as the Special until you get down below about 1.5-2 lbs, at which point the Special takes over.