I have to agree with Tony, the 100-400 is the only real choice bewteen the two. That said if I were buying I'd opt for the 400mm F5.6 as Daryl suggests, I doubt you would ever want anything shorter for surf photography, plus the 400mm prime is lighter and marginally sharper.

Having long glass isn't the only thing you need, if you're not already using one, a good tripod and head is a must have.