Spellsword is the style I adopted for my character, and what I've learned along the way is;
Restoration is, for very obvious reasons, very useful.
Illusion is, perhaps, best of them all. Nothing better than making your enemies fight amongst themselves while you clean up, or simply removing enemies from the battlefield all together by pacifying them. It's great crowd control.
Destruction is good fun (IMO) but generally not that useful (though the Cloak spells help to increase damage output when you're in melee range, which you will be with the sword).
Alteration is kinda meh IMO. If you're wearing armour (which you should be) and it's enchanted (which it should be), then really the only spell it's got going for it is Paralyse. Granted it's an awesome spell, but I don't think one spell justifies spending perks. Just calm them instead (again, Illusion!) if you don't want them to fight.
...Always look on the bright side of life, doo doo...