which fortitude cards to use

Perhaps you should be giving us deck lists so that we can properly answer your question?

Context is crucial. A deck list and an assessment of your metagame would help us immensely. I second the request for more information and a deck list.

Assuming your using lots of taste of vitae and slake the thirst like you should with blood brothers, your main priority with prevent is to stop your guy from going to torpor, Armor of Vitality is probably the best one for this as it works just fine on basic already, or if you see lots of potence hulk smash decks then skin of steel instead, though just on general basis without any specific metagame in mind, would use Armor of Vitality for its overall versatility.

I don't play a lot of fortitude combat, so take this with a grain of salt. I second the idea that Resilience is a good one because it works vs both agg and non agg. If you plan to do multiple rounds of combat, Hidden Strength is probably better to give you an extra press. If you face a lot of bats, Unflinching Persistence could be key to getting into grapple range. I've even used Skin of Night because you can cycle it no matter what. S:CE? Fine, I treat your agg as normal. Other damage? Take your pick.

Skin of Night
Combat
[for] This vampire treats aggravated damage as normal damage for the remainder of this round.
[FOR] As above, and prevent 1 damage.

I've found a mix of Hidden Strength, Resilience and a few Rolling With the Punches to work the best. They're all free, or at least "free," since they'll always save you blood in one way or another.

That's with a crypt of mostly FOR guys, though; with the awesome bloodgain and multiacting options available to the Blood Brothers, I'd probably skimp on damage prevention to focus on bloodgain instead. If you're playing a very fighty deck and one of your guys goes to torpor, your rescue action probably won't be blocked. Since bloodgain is nearly always useful and damage prevention is always conditional, I favor the former over the latter whenever possible.

Kushiel wrote:

with the awesome bloodgain and multiacting options available to the Blood Brothers, I'd probably skimp on damage prevention to focus on bloodgain instead.

Well, prevent + disarm is still a strong option, which in my opinion is less easily foiled than torn signpost + disarm (or even torn signpost + trap ).

In term of reliability, Armor of vitality can be amazing. It work against thoses pesky carrion crow ; it work against outside the hourglass + domain of evernight ; and most of the time it will work exactly as well as rolling with the punch. It don't work against celerity, so I would put mainly Skin of steel (and/or rolling with the punch if disarm is not the main theme ) with some armor of vitality for emergency.