I would say that the premise of those are quite different. While both are hobby OSes at this point, my impression is that Haiku is more of a "product" something that the developers want to provide to a general public. Serenity is written just to tickle to itches of the contributors, accepted by the judgement of Andreas. At least from the inception, I suspect that the OS was not meant to be used by people who also don't want to work on the code. But ofcourse motivations may change.

From a development perspective I think this leads to Serenity building just enough of a foundation for the application/feature someone want. When the ad-hoc code starts to smell, clean up and abstract. Then repeat.