Hey
Whether you zigzag or not is really insignificant, as long as the general trend is progressively overloading throughout the entire cycle.
As for the am/pm workout vs. 6x a week, this depends on what you actually plan to do. Assuming that reps remain pretty much the same, count the number of sets you will be doing per bodypart on your am/pm split versus your 6x a week workout.
For example:
1) Your am/pm split is arranged in an upper & lower body split, you end up having 3 sets each for upper body exercises in the am, and 3 sets each for lower body. 3 sets per upper/lower body x 3 workouts a week = 9 sets per upper/lower body exercise, which means 9-18 sets per body part (9 sets for bodyparst with only 1 exercise, 18 sets for bodyparts with 2 exercises)
2) Your 6x a week workout incorporates only fewer sets per wokout to accomodate much more frequent whole body training. So each workout, your whole body gets 1 good set per exercise. So in a week, each bodypart gets 6-12 sets.
Following the sample scenario above, you would probably be better off with the am/pm split because it allows you to train each bodypart more, assuming the setup allows you to rest adequately and you do not compromise your ability to keep up with the routine.
That's just an example. You might find in real life that the difference between the two isn't that significant when you take into account how to balance training frequency and volume for yourself in order to avoid overtraining. Also, remember that you need to eat more as you train more often, that's an important consideration you need to take into account.
Regards,
-JV