Harry, because I find his base personality and life experiences can be taken in so many different directions, whether I am reading or writing or drawing. :D
Severus, because I love tall, dark, and snarky types. Actually tall and snarky are the base criteria, but.. xDD Also, his personality and reasoning is pretty set by the time we get to know him, so he tends to be the stable point, not as flighty or easy to bend to so many different extremes as Harry.
Both, because together they make something wondrous. :D
(Though to be fully honest, I prefer Harry more. xD)