With a new player, trying to compete at tier VI is virtually an in-game death sentence. You're trying to compete with players that typically have 15,000+ battles of experience and knowledge. They are going to roast you alive over and over. That's not fun. World of Tanks is not some easy bang, bang shoot 'em up game. It's a very deep, complex game that is akin to the complexities of chess. Trying to play vs. others so vastly more experienced, it's tantamount to a high school football player trying to compete in the N.F.L. I'm afraid your tier rushing. With every tier up, we face better and better players. If we don't have the equivalent experience and knowledge, we will not be able to compete, just be cannon fodder, as well as being a detriment to our teams. Playing at tiers we're not ready for yet, we'll get pounded again and again, become frustrated with losing most of the time and quit the game. Over my 71,000+ battles and five years in this great game, I've seen this happen thousands of times. It doesn't have to be this way. Play lower tiers and you'll win more often, having more fun and learn the game at a faster pace without having to unlearn very bad habits.
As to the specifics of your question, both guns for the O-I are tier VII guns. One is the Type 92 100 mm gun and the other is the Type 96 150 mm howitzer. Being a much larger gun the Type 96 can potentially inflict far more damage than the much smaller Type 92 gun, especially when firing HE rounds. Neither gun is, "better", than another, they just play very differently. The Type 96 gun though, being a bigger gun, has a much longer reload time, so that is why the W.o.T. rating is lower than for the Type 96 gun. I personally prefer big guns firing HE rounds, as they don't have to penetrate in order to inflict serious damage upon the enemy tank. All we have to do is just hit that enemy tank anywhere and the huge explosion of that beautiful HE round can set fires, knock out turrets, tracks and injure crew members. If you can penetrate an opposing tanks weak point with that big HE round, it will do extensive damage and sometimes even one-shot that tank. Another benefit of big guns firing HE rounds is that HE round will inflict the same damage at a 1 meter range as it will at a 500+ meter range. With AP rounds, the further away the targeted enemy tank is, the less and less likely that AP round is to penetrate and when it doesn't penetrate, well then it doesn't do any damage at all of course. HE rounds are very efficient and inflict damage 99% of the time. 
Here is the Tanks.gg link for the O-I comparing both guns; https://tanks.gg/compare/o-i?l=001120&t=o-i_101125
I hope this information helps you out. 
Edited by Whistling_Death_, Oct 15 2020 - 03:30.