As far as the drill chuck - I chuck end mills in a drill chuck for counterboring only. Drill chucks and side loads don't like eachother. There are only three points of contact where a collet offers a lot more rigidity.
As far as the chuck extender - use when required but keep the length as short as needed. The farther away from support, the more chatter and deflection you will get and rigidity is of paramount importance when machining. Thisgs go wrong fast at 5K RPM.
Scott
Consistency is a good thing....unless you're consistently an idiot.