The major difference is that with a keypad you can type at your own pace, while with a dial you have to 1) wait 2) a variable amount of time for the rotary to g

The major difference is that with a keypad you can type at your own pace, while with a dial you have to 1) wait 2) a variable amount of time for the rotary to go back in initial position for each figure. Furthermore keypads allow "muscle memory". I'm not old enough to have used extensively rotary dials but I think it's very hard to form muscle memory with rotary dials. As an aside, I don't think the difference in speed the two is "tiny".