I would use the largest diameter (3/16) monel or SS given that it is probably the highest loaded control line on the boat- maybe aside from maybe the forestay/mainsheet when sheeted in maximum. Even better to use backing plates on the rivets if you can get them up in there after taking off the foot of the mast.
I never use monel because the stuff is so much of a pain to drill. I use alli and have never been let down by it. You are talking sheer strength here and anything more than alli is over engineering and if you do need to remove or replace causes monel problems.
