This has been a point of contention for me for years. I've found that the sunfist is the hardest punch for your opponent to use against you compared to a full twist like a boxer punch as when you use the full twist and turn your fist 180 degrees over, getting striked upwards by someones hand can be potential harmful to your elbow, whereas with a normal sunfist you won't encounter this problem as much as you would just have your hand come back in a more natural way as you can retract your elbow more.
Turning your fist 90 degrees in a punch seems like it would add more power from the twist but at the same time i feel it loses its speed and the little extra defense you get for it.
Personally, I feel that sunfist should be your opening attack and can continue to be every punch afterwards if you so choose to do so but I think it could become more effective if you have the turned fist be a subsequent attack as you hopefully have more a chance that you won't have to worry about it being striken. But that's just my theories on it
