This block has actually bested both the XT and the Heat Killer it seems. Though I would suggest all three honestly. Pick whatever you think looks the prettiest haha. There will be a decent change in temperature and flow rate because of the designs of these newer blocks. But if you are happy with your temperatures there's no reason to upgrade. But if you are going to switch to newer technology such as i7 definitely get a new block.
And as for what makes the new blocks better, technology has changed. They are able to create much small pin matrices, allowing more surface area, thus more heat transfer. They have also just learned from experience in general, where to put inlets and outlets, size of the area inside the block, all sorts of things to increase overall efficiency.