whatever fits the situation.

Find two sconces you LOVE, and see whether or not you have appropriate room.

Sit in front of them, do they clear your head? the window treatments? ... work with the trim, paneling, wall treatment?

If you actually intend to use the window seat to read, install sconces on the "inside" of the nook, with good wattage, and low enough to provide good reading light. I had a setup like that at my last home and I would sit with my back leaning against one of the bookcases. The sconces were low enough to give good light, but not so low that my head cast a shadow on the book.