when it comes to saltwater the real answer is always titanium (ive learned this lesson the hard way)
, a single failed element can wipe the entire tank sub 24 hrs depending on whats inside it, the reliability of an outside probe where you want it checking temps and titanium is just hard to beat. as far as wattage, you wont need alot, 50-100, really depends on the ambient room temps, if ur house is a ice box go for the 100w