1.) Is it better to do all your sets at once on the same activity or is it better to switch on and off between sets with different exercises?

2.) Is it better to do a muscle group once a week like some people do or is it better to do groups every other day. Say it's a workout schedule or 3-4 weeks.

So in other words is it better to do like:

Mon: Just chest
Wed: Just arms
Fri: Just back and shoulders
Then repeat next week
or
Mon: Chest, Biceps, etc.
Tues: Triceps, back, neck
Wed: Same as Mon
Thurs: Same as Tues

Thanks!