So some time ago I bought the E100, when I did it, I saw quite a few people saying that the 128 mm gun is better on it, however when I actually tried playing it… I couldn't make it work. I took a break from the tank because I just didn't want to play it, but recently I tried the 15 cm and… It's so much better. I don't know why but the gun is treating me very well, I even got some really dumb snapshots with it, and don't have problems with pen (though I do still fire a fair share of gold, but I don't just press 2 at the start of the battle) To me it seems the 128 doesn't fit this tank, as it's just too slow to capitalize on the dpm to chase down enemies, and since your armour isn't impenetrable and you're just a bit too slow to react quickly, trading shots just feels like the way to go.
What gun do you use? I've seen a fair mix of both in battles.
Source: https://www.reddit.com/r/WorldofTanks/comments/wu6f3f/e100_15cm_gun/