I didn't notice that this thread had been bumped!
Sentiment seems to be that language complexity is too ambiguous to quantize easily. (Of course this makes the "equal complexity" claim itself undecidable or even meaningless.)
A working definition might be based on how long it takes someone to become conversant in the language.
Which someone? I.e., how could you control for the many extraneous factors that might affect how long it takes, such as how motivated a given learner is, how much help he gets, which languages he already knows, how much the native speakers he talks to switch languages in response to his clumsy attempts, and so forth?