Sean, that's disappointing that your slab cracked.
I've warned many members in the past about using cracked stones. Basically, when a stone cracks, the forces involved may not create a clean break, and you increase the chance for chips and shards to break off of the cracked area. If one of these shards ends up in your pizza and you or a guest bit into it, it will break a tooth. I think the probabilities are very low that something like this will occur, but I don't think it's worth the gamble.
Now, soapstone and cordierite might be a little different in regards to cracks. Cordierite is a much more homogenous material, so when it does crack, the chances are greater that it will create weaker areas around the crack, whereas soapstone has veins that might, to an extent, be pre-weakened, and might fail a bit more evenly. Maybe. Did the stone crack along a vein?
I think, if it did crack along a vein, your chances for structural integrity are considerably higher, but, no matter how high the probability, you'd still have a risk. Personally, I wouldn't chance using it on a regular basis, but I'm a bit of a worry wart, and the fact that I have a chipped tooth from a pizza stone chard doesn't help.
I'd have absolutely no problem using it once or twice until I was able to find a replacement, though. I might take a magnifying glass and look for other cracks, and assuming I couldn't find anything, I'd use it this Friday, perhaps keeping a watchful eye on it during the bake to make sure no pieces were coming off.
Beyond Friday, though, steel is the answer. If you're happy with your bake times now and can comfortably hit between 550 and 600, then I think you should be able to match what you're doing now with 3/8" steel- at least, for a couple pies. 1.25" soapstone is going to better 3/8" steel when it comes to recovery. How many pies are you doing at a time now, and how many pies do you want to be able to do? If you are baking lots of pizzas at a time, I think 1/2" might be the better choice.
At least, 1.25" soapstone will store more heat initially. Soapstone will let you bake more pies before requiring replenishment, but steel might replenish a bit faster. If you need, say, 4 pies, back to back, then soapstone will work better than 3/8" steel, but, if you can, say, put 10 minutes between each pie, 3/8" steel might be a bit more responsive to the heat coming in from the burner below.
