I use both the 21st century and Sinclair mandrel dies and as best I can tell, they're all pretty similar and mostly cross compatible in terms of mandrels. 21st century, at least in the common calibers, also offers mandrels in half-thou increments, which is nice depending on how much tension you want and springback you get. The carbide and coated mandrels definitely reduce the need for lube.
But I think I heard Lee is back to making custom collet dies again? They have done a few for me, they'll give you extra collets in different sizes, etc.
