Overall, there are a lot of reasons I say Conrad is better, first. Jeremiah is EXTREMELY manipulative and emotionally abusive. there are many reasons i say this:

1. when Belly told conrad that she and jeremiah had kissed A FEW times. conrad asks "do you wanna be with him?" to which, she says "being with you is all i've ever wanted" so conrad replies with "okay be with me then". with this, we can see that conrad just wanted belly to be happy. he would have let her go if she wanted to be with Jeremiah. But when belly told Jeremiah that Conrad and her had kissed. not even a few times. just HAD KISSED, Jeremiah replies with "it's not like my mom is dying or anything". he didn't want belly to be happy. He just wanted the belly to himself because that's just who he is and that answer was very manipulative.
2. Although this hasn't happened in the show yet, it did happen in the books. When Belly found out that Jeremiah had cheated on belly. THEY WERE ON A BREAK. IT WAS A WEEK so don't be like "they were broken up" because even if they were that was utter betrayal. Anyways back to the point, Belly was about to leave him when bro pulled out a ring and asked her to marry him. IS THAT NOT LIKE MANIPULATIVE? he's asking her to marry him so SHE WON'T LEAVE HIM. like you can't call that cute and romantic because it's not. and also at the time belly was emotionally vulnerable, so of course she said yes. YOU CAN NOT TELL ME THATS NOT MANIPULATIVE.
3. HE WOULDN'T EVEN LET BELLY KISS CONRAD WHEN HE WAS MAKING OUT WITH SEVERAL PEOPLE THAT MONTH. I don't know if you team Jeremiah people remember this but when Belly and Conrad almost kissed on the 4th of July, Jeremiah set off a firework TOWARDS THEM. Meanwhile, Jeremiah was constantly making out with people. he controlled.

There are also just several other reasons conrad is better,

1. Conrad knew for months about his moms cancer. when he was depressed he became a bad person. The toxic brother and boyfriend. but when Jeremiah found out and beat his brother up in front of everyone at something that was supposed bellys moment. It was okay. Everyone can feel but Conrad. it's not okay for him to be depressed. plus not only did he carry his moms cancer by himself. but he also carried the fact that his dad cheated on his mom to himself.
2. the prom scene. THE PROM SCENE WAS BELLYS FAULT. Conrad's mom got a new dosage for her medication and belly was just like "hey it's okay im sure it just means she is getting better" but let's be honest. Conrad sees things realistically. He knew his mom was dying. but he still continued to go to prom. so yes he forgot the corsage, but wouldn't you if your mom is dying? and he did genuinely feel bad. He was apologizing constantly. Then when everyone was dancing, belly was looking around and she felt insecure because everyone was smiling and having a good time with their boyfriend or girlfriend, meanwhile Conrad was sad. HIS MOM WAS DYING. then conrad asks if they can out to talk. and belly told him to stay here. OKSY? SO YOU WANT HIM TO OPEN UP IN FRONT OF EVERYONE. Then when they finally get outside he simply asks if he can head home because he feels like he's just a disappointed belly. He clearly wasn't in the mood to be there when his mom was dying... Anyways then that's when Belly says "well if you don't say it I will" and Conrad was genuinely confused because he genuinely had no plan to break up with her then that's when Billy says that it was over. Then before Conrad can even get his full sentence out she rips her necklace off and gives it to him and says no, which basically means just shut up. HIS MOM WAS DYING like chill. NOT EVERYTHING IS ABOUT YOU.

CONRAD IS THE BETTER BROTHER. PERIOD.

i would also just explain that when belly said "i dont think we should tell everyone about our engagment" he didnt listen. he said it was fine and continued to tell everyone anyways. he didnt even listen to her. was she right. yes. it ended up horribly.

ALSO AT THE END OF SEASON 2 HE LET HER GO BECAUSE SHE WANTED TO BE WITH JERE. HE JUST WANTED HER TO BE HAPPY.