when would you choose one over the other? Thinking of spooling my main shatter bait rod with 40 lb this summer for grass fishing.

Doesn’t  really matter. I never use 40. I use 30 or 50. 30 for some of my moving baits and topwater, 50 for heavy grass and frog fishing. 

9 minutes ago, basser27 said:

Doesn’t  really matter. I never use 40. I use 30 or 50. 30 for some of my moving baits and topwater, 50 for heavy grass and frog fishing. 

I use 2o for small cranks and topwater, 30 for pitchin, 50 for frogs, buzz, and paddle tails, and 65 for flippin

I've never used 40.  I use 20, 30, 50.  I do use 30 for jigs and chatterbaits. 

   I never paid attention to test on braid. By far and away the most important characteristic for me was the manner in which the line would cast, would not backlash, and would exhibit knot integrity. That was chiefly related to diameter, but there were secondary characteristics involved also, such as flexure  and friction.

   I've landed pike on 4 and 8 lb. mono, and musky on 10 lb. braid and 12 lb. mono. Any reasonable braid that will cast off a baitcaster has greater tensile strength (test) than I need to worry about.     jj

Most 40lb braids have a diameter equivalent to most 12lb mono.  To me, that's the sweet spot for casting. I use 40 a lot for pitching, jigs, swimjigs, buzzbaits.  I've used 30lb for topwater but it's not my favorite. 

 I believe you’ll be fine with either 30 or 40lb.

 I use 20 for chatterbaits and various Wakebaits/topwater lures.

 I use 40 lb for frogs.

When fishing with braid I mostly use 30 pound test and use 50 pound test if needed.

I use .013" diameter braid or .33 mm for those who prefer the metric system. In fact, I use this theory when selecting mono, fluoro or braid. Those numbers are about the sweet spot for me, but .012" or 31 mm also work well. I am not concerned with a manufacturers "lb test" as much as I am manageability. Proper drag setting, maintaining line, re-tying, knot tying, etc. are all factors that determine how well your line performs.

I like 40lb 832 which I believe is 10lb diameter.  I use 50lb on some rods.  30lb is a little thin for my liking.

40lb smackdown and 40lb power pro maxcuatro. Size of standard 30lbish