To answer question 1 if both your strikers have the same value and quality, i'd make the decision based on which player had the best match stats and/or scoring record

The second question is like asking how long is a piece of string. there's no right or wrong formation other than once you chosen 1 you'll sign players to fit in to that system. I've got 4 accounts and every team plays a different formation i don't really have a favourite. My main team has a squad that lets me change to nearly any formation possible so i can adapt tactics to who i'm playing.

To be honest though i only change formation when i'm playing a much stronger team when my tactics are usually defensive looking for a smash and grab victory