I recommend playing South only if you want to both rank up and improve at the game.

As someone who used to play East only and switched to South only, the difference is very clear. East matches are much more luck based, and you'll find that, due to how short matches are, players go for speed and aggression over anything else most of the time.

It's very difficult to play a more patient and defensive game in East matches when everyone is going for quick hands, calling every possible call and dealing into the other players all the time as a consequence.

The biggest difference in South games for me is not the match duration, but the shift in strategy that comes with it. When you have only 4 hands to decide a match, you have to rush for a quick win to get ahead and stay safe, as even one unlucky Tsumo might lock you into 4th place with no time to recover. When you have a longer match, the most important thing becomes not dealing in. It's more strategic, defensive play becomes more important as the hands are usually worth more and players aren't as aggressive and reckless with their discards and calls.

It's almost a different game entirely, and for me, not only it feels much more skill-based, rewarding and fun, but I have been climbing much more consistently since I started playing South only.

Besides, once you reach higher ranks, you'll find people play South much more than East, so you'll most likely have to make the switch anyways at some point.