Hello,

 I do not have any reference links but I have attended many SharePoint Saturday events and I have had some time to talk to both Nintex and K2 sales reps and their techs. The main difference that we understood was Nintex is built on top of the SharePoint workflow service. K2 has its own workflow service. It took us quite some time to make a decision on whether or not we wanted to purchase Nintex or K2. We went with K2 because the workflow service was seperate from SharePoint. We can also build forms and processes outside of the SharePoint environment with a SQL backend. This was not something available to us with Nintex at the time. We viewed Nintex more like an enhanced InfoPath. K2 seemed to give us more options in terms of development. With K2 we can develop the processes outside of our SharePoint environment so we do not have to worry about 400 processes breaking on the next SharePoint upgrade. We simply post the links in SharePoint and the users do not know the difference.

P.S. K2 has a lot of bugs. Hopefully one day they will work them all out. There is also some functionality that should exist but doesn't.

I hope this helps to answer some of your questions. Feel free to message me if you have any questions.

Thanks,

Bryan Peters