I happened to make a previous comment about this.[1]
To add some observations.
Equal split can work if the cofounders have known each other for a long time and have already worked together on a previous project. They have the history to confidently gauge each others' future work dedication. An example would be family siblings. Another example is Larry Page & Sergei Brin of Google. When they cofounded Google Inc in 1998, they had already met in Summer 1995 and worked on the "Backrub" research (precursor to google) for more than 2 years.
Unequal split may be more prudent when the entrepreneurs haven't worked together much and therefore haven't tested each others' resolve and dedication to a startup. A famous example would be Drew Houston and Arash Ferdowsi of DropBox. Drew needed to add a cofounder in a hurry and a friend helped him connect with a possible candidate. In this case, he gave cofounder Ferdowsi a smaller ~25% instead of 50%. Other examples of unequal splits include WhatsApp, Instagram, Youtube, Microsoft.
Every situation is different. Robin Chase (Zipcar) thought she was in the 1st scenario of equal cofounders giving equal effort when she was really in the 2nd scenario of unequal effort. That ignorance led her to regret her decision for equal split.
Another generalization I've noticed is that more experienced founders that had equal splits in their 1st startup prefer unequal splits for their 2nd startup. I can't think of any famous founders that went the opposite direction of unequal to equal.
For the financial split, the goal is to come out with something viewed as fair and reasonable. However for decision making, it is critical to have some way the company can make effective decision. If, for example, the co founders are on a sailing ship suddenly see rocks dead ahead, a decision must be made whether to go to the right or the left. The worst thing is not to be able to decide because two co-founders/co-captains have equal shares/voting and there is no mechanism to effect the decision.
Further more, if two (or more) co founders cannot decide before hand on how to deal with a deadlock, then it is always better to quit while you are ahead.
And yes, I had this experience as well as one that was the opposite.
