If you wait a month, we would have something super sexy to help you decide.
But in the meantime, the general advice is whether or not you are looking for flavour in your army, or combat effectiveness. Granted that some formations would grant you both, but some superheavies just don't have that option to them.
Baneblades and Hellhammers give you tons of weapons to play around with, allowing you to take on all foes. The former also has the option to take different crew, especially important if you plan on fielding an infantry heavy Apocalypse army.
The Shadowsword variants are high specialised in what they do. As mentioned, the Stormlord takes care of your anti-MEQs needs very nicely. The Banehammer is especially useful against fast armies, giving you a few more turns of shooting before they reach your lines. The Shadowsword does a nice job to take out other tanks, superheavy tanks and the odd Titan, and even has the formation to go with it (ie. the Shadowsword Domination Force that I believe you are far more than capable of fielding).
In the end, it is your choice of what you want. I know that I got a Hellhammer because it's part of an urban assault force, so think about it. Flavour, or effectiveness?