some floro's put out more lumens/lux/par per watt. here is the best comparison and explanation i know of:
t5's(high output or compact floro's) and t8's will usually be more efficient than t12 or t12HO bulbs.
another example, i think, would be that larger versions of bulbs (say a 96W compact floro bulb, vs. a 36W or 55W) will be more efficient and produce more output for the watt.
also keep in mind a bulb's ability to be reflected and increased bulb temperature can drastically affect their output.
lot's of variables...