Either tank is a tremendous points sink, but if you can only have one, go for the Baneblade. The Hellhammer is very short ranged (36”) with its main gun and the longer ranged weapons (lascannons and hunter-killer missile) only go out to 48” and aren’t much of the threat to the things you want a super-heavy tank to be dealing with.
Neither the Baneblade or the Hellhammer is a reliable “Titan Killer,” although they can be pressed into the role if needs must. Where they shine is dealing with Knights, Greater Dæmons, and heavy tanks (e.g., anything that used to be AV14). The longer range of the Baneblade main gun (72”) often lets it hit those targets from further away than they can retaliate.
Sponsons usually aren’t worth it, certainly not two sets of them.