I have a 9 oz. poly mallet and a BK 16 oz. tapered mallet in my kit. I do just about everything with my BK mallet. Now I haven't used it on a strap end punch but I have used it for setting snaps. rivets and oblong punched. Just give the tool a couple of good whacks and keep moving on. I would recommend to not use it on your smaller seeders because you could easily drive it straight through the leather into you slab and ruin your seeder, that's why you have a smaller mallet.
Don't underestimate your maul. Even a 14 oz. one can pack a good punch.
Now, when to use one over the other, I think that for the most part that's personal preference. I use my heavy mallet for almost all my tooling (except for my small seeders), I use my small mallet when I'm using said seeders or when I'm tooling and I don't want to make too much noise, my table that I tool on amplifies almost any kind of noise from hammering to dropping tools on the table.
Just experiment.
Damon