.357 vs .45 , which one and why?

If you are just planning coyotes and pigs, then 357 is fine. Keep the shots close and only shoot med sized eating pigs. 45 will be more effective than the 357 on double lung shots, but if you can put it in the brain they don't take another step.

thats right !!! even when using JSB pellets to the brain.....its over !!

my standard SL stacks them pellets very nicely !!
 
I have a Bulldog in .357 making 335fpe with a 190g slug and a Texan SS making 500 fpe with a 372g slug. They both have adavantages/disadvantages. The .457 has a bit better BC and knockdown and the Bulldog is more accurate within 100 yards. I reach for the Texan a bit more that the Dawg, but it is heavy and a bit more awkward in a stand. The Dawg is my brush gun and I really don't see much I wouldn't point it at.