Sorry I've been out all day (relative celebrating his 50th) and saw the tag by @gr5 now, but looks like you got it sorted out. These days we don't take these apart when replacing belts for customers, we use pre-assembled kits instead, saves a lot of time and the cost is not high enough to justify the extra time it takes (while risking cracking the block and having to replace it anyway).
As for which is better or worse. All sliding blocks can crack if you're unlucky I'd say, but the latest ones, the ones you have, that are white and have internal tabs seem to be very robust. The first white blocks (with external tabs) that came out for the UM3 were pretty bad, lots of cracked blocks there. They got replaced by black ones that fared much better but these were then replaced again by the new white ones. It's a bit of a mess as far as part numbers go there 😄