When you become christian you can still train warrior monks and your troops still suffer morale penalty from enemy warrior monks despite what the manual says. However you will get significant unrest in any province with a buddhist temple in it and the rebels will include warrior monks in their ranks. The morale effect from guns can be quite devestating though in original shogun neither kind can fire in the rain so the are not as good as in MI. You can get a lot of extra koku from becoming christian, each trading post is 200 extra koku and once you get a cathedral you will get 200 extra koku for each church you have...which can mean a lot of extra koku.