tough question, because there is a lot more to it than pick A or B

do you want to have fun or win? I'm gonna go with you want to win because you asked which team would give you the best tourney results. Magneto is always the best option if the other option is Wesker, because magneto has no ceiling and you can take him as far as your skill will allow. Either of those teams can compete but will they give you optimal tourney results? It depends on match ups and it depends on how good your raw skill is. Be honest and tell yourself you aren't justin wong and chances are you never will be, but that doesn't mean you can't work towards the goal of being the best with a sub-optimal team. Magneto will win more match ups on paper than Wesker will, this is fact, so the chances of doing better in a tourney will be higher with Magneto, but only if your skill isn't off on the day of the tourney because Wesker has easier execution and doesn't take as much work to compete with, but ultimately suffers from more problem match ups than magneto.

Team A is the weakest, but could be made better by swapping out Dante with Doom. You can stick with Dante but utimately I like Doom because he's more tournament friendly, can do a shit ton of damage and has an easy TAC infinite. Yes you can do damage without needing to TAC but with wesker, there will be a lot of opportunities where damage is left on the table if you don't TAC. So you can play this team with Dante but learning the Dante infinite is an absolute must, and so is learning the Doom infinite if you picked him instead. Doom > Dante simply because plasma beam is easier to work with and the doom/strider damage is very good that should almost never require a reset to get a kill. There are pros/cons to both Doom and Dante with this team, but that's something only you can decide what works for you and what doesn't.

Team B is better but needs optimization. Either drop X23 for Doom or drop Ammy for Dante and you have an A+ Tier team. Mag/X23/Dante for 300% dirt nap game ending wins, this team relies on TAC infinites to build 5 bars then set up unblockable ToD's on the next 2 characters. There's a really strong flow chart associated with this team, I can only offer the advice of asking /u/merkyl999x for help. Mag/Doom/Ammy is super top tier and is one of the best teams no one plays. It's really Mag/Doom then Doom/Ammy, so look up any top Mag/Doom or Doom/Ammy player for knowledge on how to use these shells, any tourney results will tell you these are top tier shells and have great chances at taking tournaments. In the end though, these magneto teams will come down to execution and skill so consider what youre ceiling is today and what you want your ceiling to be months from now.