You'll probably get the best answer from Milkmaid.
However, my view of the two:
Friesians are about volume, volume, volume (of milk). However, it is at the sacrifice of some taste. They are great big buggers and as such will eat more. They tend to be more flighty. That said, the friesian is really the only breed to have if you are wanting to do dairying full time, economically (here in Australia, anyway).
Guernseys are a slightly smaller cow, and so may be a little lower maintenence in terms of feeding. They will have a creamier milk but wont produce quite as much. In my experience they are slightly quieter. I have never milked one, so am not sure how they go in that department. In Australia, the guernseys have very little foot problems, whereas most of the friesians have terrible feet and legs.
What are you looking for? A house cow, nurse cow or commercial dairy operation?
Might I suggest for a house cow a jersey (creamy milk, small cow so low maintenence) or if you want a bit more milk an Illawarra?
For a nurse cow or in a commercial operation I would stick with the friesians. The large quantity of milk will allow up to four calves to be reared on her, and if in a commercial situation they are the only ones that really make money.