To confuse matters more, dichlor also comes in different strengths, I've seen 52%, 55%, 56% and 62%. the higher the strength the less chlorine is used to get to the PPM you want.
Of course you have to measure the $$ difference to the amount used - which one be more economical in the long run or will they equal out.
Fritz - As far as I know only Emerald Spas says it's OK to use trichlor in their tubs. They have a feeder system in their filters to feed either bromine or trichlor.