You asked how to determine which stat is more important, so I'll answer that.
- Check the Pokemon's base stats; if it's base defense/or special defense is not usable, it is best to not use it in most cases (except Chansey/Blissy)
- What tier is the Pokemon in?; this will help determine which threats are more common, physical or special (if you do an in-game team, use the common Pokemon in that region)
- What type is the Pokemon?; if threats to it's defense, for example, do /4 damage, it might not even be that powerful, and you may have a better time investing in, for example, atk.
Which moves are special/physical don't always matter since they're not all used (for example, no one will ever use ember, tackle, vine whip, etc.)
Hope this helps, but it is hard to specify without an exact Pokemon to base an answer off of. Also, some Pokemon will never use defense (Sweepers, like Tapu koko and Pheramosa).