I've worked out joint will be best for us becasue OH is self-employed and only works term time. A few people have said tho that they called Revenue and revenue worked out the best deal for them, they were really helpful and good.
Revenue will have worked out what was most efficient for them but as long as you are jointly assessed there are no additional savings that they can magically give you.
They would have worked out say that one member of the couple earned below the standard rate band and given the balance to the other member. This means you get the additional money month to month which is efficient. However if you didnt work out the split and then post year end did a P21 balancing statement you would get all the unused band back in one lump sum.
Being jointly assessed is only useful if one of you earns less than the standard band or cant use all their tax credits for some reason and allows them to be transferred.