Cassiopeia or Syndra ?

I seem to have to been cursed with loving weak champions. It was Azir first(before nerfs), then it was Kayle.

I enjoyed both these champions a lot. Kayle even got me to Silver. However, I noticed that these champions have a number of glaring weaknesses that meta champions do not have. Azir, unlike your typical mid pick, think Zed, Yasuo, Velkoz etc, doesn't have the early damage and can't really wreck anyone in the 20 minutes of the game without some major outplays. Kayle, like Azir, doesn't really do much in the early game either. She gets out-traded by almost every champion in the game so she has little to no kill pressure in lane. Her roams, unlike popular meta picks, are lackluster. Unlike Yasuo who could roam bot lane and pick up a tripple kill with nothing but a Doran's, Kayle cannot do this. She needs at least a Nashor's.

Now I was able to handle myself with these champions but that took a lot of effort and a lot of games and I know now why these champions aren't picked much when compared to the likes of Zed, Yasuo, Zyra, Brand, Jinx, Caitlin, Vayne etc. It's because it takes a lot more effort to learn how to do well on them. That is not to say the popular champions are easier. They simply have fewer weaknesses to overcome.

Now as the season has ended, I started playing some Cassiopeia and I enjoy her. I could see myself maining her in Season 7 but I don't want to make the same mistake of maining weak champs or champs that are likely to get nerfed into weakened state like Azir. I'd main someone like Yasuo or Ekko if they weren't so damn popular. Cass isn't all the popular and I enjoy her so she's a good contender.

However, after a set of games on her, I noticed already that her damage seems to be underwhelming compared to top picks and certainly not up to scratch when compared to typical mages. I've faced Velkoz several times now and got completely destroyed owing to his far superior damage. For the time being, I'm content to accept that I'm just a terrible Cass player but I don't want to wait till I have 200 games on her to realize that she just not as strong as a typical mage.

So my question. Is Cassiopeia a good champ to main ? Can she stand up to all the top meta picks ?

Another contender is Syndra. I enjoy playing her too. Now I know Syndra is really strong. However, I understand that she has a high ban rate in high elo which means nerfs are probably coming. I don't want to start getting into her only for her to get nerfed harshly later on. Happened with Azir and I hated it.

Of the two I like Cass more. So what do you guys think ?