The thinner the bit, the more sensitive. The thicker, the more mild.
HOWEVER, the palate of the horse (size of their mouth, how low or high the roof of their mouth is) is dependent on what thickness to bit to get.
French link or 3 jointed bits work off of tongue pressure. Regular snaffles work off of bar pressure and tongue pressure.
MY horse prefers a french link. He used to be in a snaffle.
The type of ring is important too. Loose ring has nothing for the horse to brace against, but can pinch (hence bit guards) or slide through their mouth. Eggbutt, half cheek, full cheek, and dee all give LATERAL pressure. Some more than others. This is like a hand pushing on one side of their face to help them with steering. Some like this aid and later transfer them to a loose ring so they don't brace.
It's up to you. Personally I think starting with a halter and rope, then progressing up to a side pull, and finally a bit would work really well.
"Strength is the ability to use a muscle without tension"