I don't know, I'd think Kaho would be more important at first.
Kaho should give you better stats out of it, it'll give you a better boost in int.
The only reason I'd possibly get a tome first is if you were going to go autocast and autocast alone.
Course this is referring to pvm situations.
If you really want to pvp, then strip shield will help out a whole lot.
But then of course you'll want both

I'd personally go with the kaho first, its not character restricted.