Revo is open source. Distribute the code for free, sell your services. You’re not selling the code. Now if you make a commercially licensed Add-on that uses ExtJS, you need a license.
jQuery has a huge amount of momentum, gets better, faster and more capable over time, and has a huge, friendly developer community. It’s a great fit for Evo in my mind. Mootools is definitely more developer-centric, and thus would alienate a large base of Evo users, but it’s an amazing and awesome bit of code. I’d say Mootools is a hybrid between the intuitiveness and simplicity of jQuery and the power of ExtJS.
These are just my opinions and not the official MODx project line. While I admittedly have a bit of say in the MODx project, I’m certainly not the final decision-maker.