I am playing with a dual-class fighter mage and have just acquired the Fighter stronghold. Does this mean that I can get the mage stronghold too? If not which one is better to have. The character is currently gaining levels as a mage so would it be more beneficial to get the mage stronghold?

If you havent installed all bug fixes it is theoretically possible. Many people mentioned that they made fighter stronghold (but didnt talk with Nalia and accepted it) then made mage stonglold, accepted it and went back to nalia to got fighter stronghold. Game does not check in all possible cases do you have already one stronghold, but I am not sure if it ill work if you already have fighter stronghold. - Good luck

2nd question: strongholds are nothing really special, you earn or spend your money, got small quests. For mage there is possibility to get some items or at the end some helper who will make potions
Zbyszek