Braum or Alistar

You can get Alistar for free through Riot's promotion on Youtube. The process is actually broken, so you'll need to submit a Support Ticket, but Blitzcrank Bot will get it to you instantly.

There are 3 you can get from the promotions that are always running. Check out the link below.

https://support.riotgames.com/hc/en-us/articles/202083184-Promotional-Content-Free-Swag-

Alistar imo. I have both, but I always find myself too reliant on my team when playing Braum. Plus with Alistar you have sustain with heal and more offensive abilities. After the mastery change I feel life is hard for pure tanks like Braum too.

Alistar is overall a more viable champion, and his quality of life buff next patch will only further his viability.

Champion.gg says, statistically, Alistar is the 10th best support whereas Braum is the 12th in overall performance. I also personally vote for Alistar, as a Flash+Q+W back into the team is a particularly good way to screw over an enemy carry.

The direct matchup page largely compares them as a skill matchup. Other than damage dealt, these guys have very similar stats all around.

Ali is somewhat harder to play effectively, but also more rewarding, Braum is somewhat niche, what I mean is Braum is almost all about peeling and disengage and it's really simple to do just that as Braum, but making a pick will be really hard unless you are ahead. On the other hand Ali will fit almost any team, but at the same time if you want to peel with Ali effectively you need to time abilities very well to break enemies dashing at whoever you are protecting, to make a pick you need to use up both your cc skills and range is not-so-great, he is heavier on mana, you need to really care about angles etc.

Ali kit is simple mechanically, but there are a lot of subtleties in it.

This is very subjective though, I only played several normal games as Braum just to never come back to him, apart from blocking some important projectile-based ability like Trueshot Barrage I never felt need of picking him over Alistar.

Play both, pick the one you like more (you can get Alistar for free, as Deep Terror Nami pointed out). I'm also a support main (sort of?), I like both. Both champions have their pros and cons, though in general I think Alistar is slightly more versatile and is more high-risk/reward, where Braum tends to be a bit more defensive.