In my experience they both are just as accurate.
I have used the piston type and wedge and IMO the wedge makes for a much more solid locking of the tools holder.
The accuracy isn't the problem that I noticed, its the piston tends to wear or dimple the tool holder and that is were the accuracy changes starts. And I believe this is what paul maybe refering too.
Although I have a KD at work wich is supposed to be the "HIGH" end wedge style (and the price is to match) and have just about worn the wedge completely out. Although to be fair the tool post is probably as old as me!
I have a phase two wedge type i got for the mini lathe and i think I overkill the thing is for a big lathe (wanted to keep the tooling I use at work) should be good for some laughs when I get it tooled up and take some pictures. and the quality is good.
I haven't used the piston type, so no comparision, but the wedge type Phase two is rock solid, and very repeatable. In a years time you won't be able to remember if the wedge type cost $30 more or $60 more. I have 11 tool holders for mine.
and that says it all.
thanks
Michael T.
"If you don't stand for something, chances are, you'll fall for anything!"