Is this a mountain bike or road bike? For a road bike the 36mm will allow you to use a 1 1/8 steerer and straight steerer forks are still common, so no problem with that. If it's a mountain bike, I would go for the 46.4 which has a 44mm ID and will work with a straight 1 1/8 steerer with both cups internal or with a tapered fork with the top cup internal and the lower cup external. Tapered steerers are the norm now in mountain bike forks, so your choices will be limited for a straight steerer fork if you go with a 36mm headtube.