|There's no question in my mind what to use. I would go with fiber.|
It might be a little more expensive but it's worth it.
The maximum segment length of Cat6 is 100 m (320 ft) so you would need at least one repeater in your 400 ft run. Multimode can easily span 400 ft without any additional equipment. Singlemode can cover even longer distances (25 km's if memory serves me).
Plus, you can get more bandwidth ("speed" is the wrong measurement) out of fiber optic. We're moving to 10 Gig in our server room and the same fiber will do 40 Gig. So if/when the time comes to upgrade to 40, we won't need to replace any cable, just GBIC's.
Finally, you could buy a multi strand, preterminated cable that would allow multiple connections. We use a lot of 5 pair preterminated fiber cables between our closets and in our server room.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.