Without nen, Killua is by far stronger, faster and overall stronger. Tough question, I've wondered about this myself, but I'm rooting for Killua. They both have so much potential and strength that it's scary to think about how strong they'll be when they grow up, but as of now I think Killua's the victor. Gon's no softie but Killua has gone through so much training to improve his strength, speed, durability, stealth, assassination, pretty much everything an assassin needs. His training was from childhood was extreme, resulting in an extremely strong fighter.