Good fight. this would be Marquez vs Diaz all over again, the only difference is that Katsidis has more pop then Diaz.
That fact alone should make it an interesting and explosive match. The winner of that should definitely unify the belts against Valero and become the undisputed lightweight champion.
That's bigger money but a far inferior fight to Marquez-Katsidis. Michael's evolving into the fighter Ricky always wished he was. As long as Marquez isn't badly damaged by the Mayweather loss (thinking more mentally than physically) - he should crush Ricky in the first half of a fight and Michael in the last rounds. A Katsidis fight would last longer and be much more competitive.
I'd pass, Kat would get picked off easily. (Why settle for Kat, Get Hatton or a 140lbs championship fight for JMM instead, why go back to 135lbs?) Go after that 4th @ 140lbs. (Go after a champion)