Where the upconversion takes place depends on the output options of the cable box. Some allow you to pass the signal through untouched, others upconvert it to whatever the output resolution is you set the box to.
As for the upconversion, most likely the TV will do a better job, I suggest you try both if it's an option and decide for yourself.
As for the upconversion, most likely the TV will do a better job, I suggest you try both if it's an option and decide for yourself.