I've used both. I love Tropic Marin salt, it's the only one I know of on the market that provides 100% of the elements in seawater. Using Tropic Marin is as close to natural seawater as you can get IMO. It's really not much more when bought by the pail, maybe $10 bucks more than IO.
Haven't quite figured out BioCalcium, it can very easily screw up your ionic balances and throw you tank outa wack requiring massive (ie 50-75%) water changes to get the ions back in order. It's not really easier than kalk since you must monitor it closely by testing alk and ca levels.
The cheapest place I found to get Tropic Marin 200 gal salt mix is That Fish Place which charges $48 plus about $15 shipping. Does anyone know any cheaper places to buy it? Eddie G