Thanks, Alexi. I noticed that you had taken the steel plate plunge. I have no doubt that you'll be very happy with it.
Presently, I'm very gung ho about steel. 1/2" steel plate outperforms 1 1/4" soapstone in every way and is much less expensive and easier to track down. The only thing that might be better is 3/4" steel, and, presently, besides being uncharted territory, 3/4" steel can be a hassle to work with because of it's weight. You basically have to replace the oven shelf with flat steel bars in order to accommodate the weight of the steel. The one thing that appeals to me about 3/4" steel is that there's a distinct possibility that, with a 45 minute preheat at 550, the 2 minute bake barrier could be broken. I know Toby (Foolishpoolish) has his Nearlypolitan, but, to be honest, I think the Nearypolitan logistics can be a bit demanding and not really appropriate for the beginner. Supporting 3/4" steel plate requires some DIY skills, but, I think, once it's in place, assuming that 2 minute bake barrier can be broken, the process will be relatively idiot proof- at least in terms of bottom browning/charring. The top will require very close proximity to a red hot broiling element.
So, to answer your question, here's is where I currently rank the stones I'd consider worth buying.
1. 3/4" steel plate (theoretically)
2. 1/2" steel plate
3. 1 1/4" soapstone
and that's it. 1" cordierite kiln shelves were a contender for a while, but 1/2" steel is about the same price and far superior. 1" cordierite, preheated to 550, probably will bake a respectable 4 minute pie, but why choose probably when, for the same price, 1/2" steel will guarantee it?