I don’t have a lot of experience but, I have used both and can tell you what I’ve learned.
TL/DR: Purple for fine details/small things, but needs optimized burnout for any worthwhile results
Blue for larger items, Hard to work with, with shrink quick, but casts nicely.
I can say for smaller and more detailed items use the purple. There can be problems with the purple https://forum.formlabs.com/uploads/default/original/3X/b/4/b4fd937adaa68c13078b305883b7fa6b5ff8fdc5.jpg Bad prints out of brand New Form 3. I experienced these artifacts this and replaced my printer to address. Outside of that the purple prints very reliably and accurate. You will need to use a casting house who is proficient with this material because you can get poor surface quality with less than optimized burnouts. I had a lot of trouble casting this myself and getting good surface.
The blue CW 40 has given me much trouble to print. I have yet to have a run without some prints failing similar to Skinrage Wax 40 or even sticking to build plate. There is a history of shrinkage to the model and gets brittle the longer it sits. I haven’t had much of a shrinkage issue myself. Getting supports off is a chore, supports must be quite large respectively for jewelry purpose, and the material is so soft you run risk of destroying fine details. I can’t stress how soft and hard to work with CW 40 is compared to to other materials (primarily the Purple CW). Casting results, however were very good. No surface issues of note. Here are others inital expeirence with CW 40 Form Labs Castable Wax 40 resin: First impressions and Castable Wax 40 Results - 10% Shrinkage Hope this was informative and usefull.