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Casual 40t mark.
Hard to say. I reckon they are equally good but one can influence a match more heavily than the other.
A lot of the people claim the 40T as superior to the 50 100. Part of this is because of how "easy" the 40T is as its clip time of 32ish seconds is more forgiving and therefore managable by most players. Consequently more people can play it to a higher level (hence high skill floor - it is hard to perform poorly but easy to do well). As woth any autoloader, you still need to know typical positions, clip time, future positioning, flow of the game and so forth to continue being effective.
The 50 100 in contrast requires you to know everything you would do in/know about the 40T but to a greater extent. The 50 100 is a specialist role. If you cannot play the tank to near perfection, you cannot/should not play the tank over a 40T. This used to be the case in WGL when 7/54 was still a thing. The people who played 50 100s could single-handedly change the flow of a match with one clip. The same concept held true in pubs. You HAVE to make sure your clips hit at least 5/6 (roughly a tier 8 hp pool) in order to be relevant. Any less and you are useless as the near 50s clip time crippled your team. For this reason it has a very low skill floor as it was highly unforgiving and very easy to fuck up. However, I believe mastery of the tank puts it above the 40t for the simple fact that it snowballs any engagement out of control.
I would think of it like the batchat 25t as the 50 100 and a batchat with 4 shells (can no longer 1 clip t10 meds) and a 30s clip as the 40t. While the reduced clip time makes it easier to play for all skill brackets, nothing comes close to the game changing prowess of a mastered bat.
40T is better for more players due to managable clip but 50 100 has a higher skill cap and is also exponentially harder to play well.