The blocks are basically the same but there are a few differences. Deck height is taller on the earlier blocks and there's some minor differences in the oil passeges and that's about all. Strength is the same for all of them.
The location of the head bolt threads are different on the 86-87 109 block. The threads are countersunk deep in the block. The engineers did this to provide a more stable deck surface when the heads are torqued in place. IMO, this make the 109 block a better choice if you have a couple of blocks to choose from. Brian is correct, the strength of the main cap area is the same.