The best part (at least for me) of a CIM that NQNS is that a lady that is NQNS will usually shove it down her throat right before you cum, allowing you pump your load hard down her throat as she is working those throat muscles to push it down. This is so intense and usually does not over stimulate the freashly spent cock!
When a lady is NQBS, she usually will keep your cock with a little more then the head in. Sure the pumping of hot cum into her mouth as she flicks her tongue is still a huge turn on, but you miss out on the deep throat action.
The only time I want to cum with my cock barely in her mouth is if she is going to spit it out all over her tits and titty fuck me with it!