How wild are you going to build this engine? A production (wrecking yard) block is more than good enough for a mild build. I wouldn't hesitate to use one if you can find a good core. As Eric said they are getting a little harder to find. I went through 4 blocks before I found the one I am running now.
A bow tie block is an excellent option if you can get one for $400.
If I had it to do over again I would have went with an aftermarket block, only because I am really pushing the limits of a production block. By the time I had the block splayed, line honed, decked, fluxed, cleaned and the rest of the machine work I was probably about half to 3/4 the price of an aftermarket block.
The rebuild aspects that Eric mentioned is also a very good point. I am at the point that I am out of rebuilds, so all he money I have invested in this block over the years is going to be gone when I blow it up (notice I said WHEN not IF).
Bottom line is how much power are you going to make? What will the car/engine be used for? (racing, street driving, towing).
For anything up to about 475HP I would use a production block no doubt, for 500HP I would start thinking about a Bow Tie block, about 550HP and up I would go with an aftermarket block