The Simon gets my vote as well - a very handsome town house with an attractive central courtyard. For something slap in the middle of the city, it's also remarkably quiet as well.
Although the Easter festivities will be over by the time you get there, if you are staying at the Simon it would still be a good idea to book now, as it fills up quickly.
Seville is a fantastic pace and well worth spending a few days before starting walking - I think the Alcazar is even more beautiful than the Alhambra.