I have always used the Lee dies powder through with the floating ball in them.  While they were ok, they seemed curl the edges as they flared more than just open the case a bit.  

 

I took the plunge and bought the Lyman expanding dies and what a difference.  The cases are expanded (more stretched open), the bullets sit in the cases cleanly, and they taper crimp (straighten) very nicely.  I think they were worth the investment. 

The "M" Die does a great job --  I have 1 for every pistol caliber I load

I noticed the PTX by hornady does the same thing... not a tradition bell... apparently people have issues loading non-jacket with the hornady ones, peeling the bullet. I haven't noticed any markings on the jackets tho... what is nice about them is that there is no way to really over expand the case adds a bit more consistency with cases not exactly the same length, and like you state the taper comes out more consistent since it doesn't have to roll the case back. I'm not sure which way is better yet in terms of case life.

I started looking at the lyman dies because I was peeling both plated and lead bullets sometimes. I felt the amount of flare was getting ridiculous. The lyman dies seem to open the mouth and not curl out the edges. Bullets seem to sit straight with no issues with less stretching of the case compared to the lee powder through dies.

 

I'll find out on the issue of case life soon enough and report back.

I'm assuming your crimping in a separate step, that's unusual to be peeling the bullet with a heavy case bell.. In my cases the only reason i wound up with these expanders was because that's just the way hornady does it for their powder drop. I was worried about it at first because they didn't get stellar reviews from the lead guys, but they work very well for me with plated bullets. The Lee dies probably expand the case a bit more.

I crimp in  a separate step.  I'm stingy on the case expansion so that's when I get shaving.  I feel like I expand the same or less with the Lyman dies to get good bullet seating.