It depends upon the age that this siege takes place. If it where the Fourth Age or anytime since then, then I would say Minas Tirith. On the other hand, if it were anytime after Helms Deep was constructed and before the hornburg was destroyed, then I would say Helms Deep.
__________________
Sing my world into being, Ainur. The universe needs your power!
I would say Helms Deep because in the time of Helm Hammerhand, the wild men of Dunland layed siege to the horn burg and the eorlingas held out for over a year before defeating the wildmen. for food and supplies they used the secret passage that only the house of eorl know. besides, only one army ever set foot inside the hornburg, they were the Urak-Hai of isengard and the wildmen of Dunland that were on the Urak's side. whilst Minas Tirith would have crumbled if it were not for the rohirrim.
