The difference between seoi-nage and seoi-otoshi is academic. In the former, you spring up when you do the throw and in the latter, you merely pull or roll uke over. But nobody really goes into the technique thinking: "I'm going to do seoi-nage" or "I'm going to do seoi-otoshi". Whether you spring up or merely pull uke over depends on a lot of factors, chiefly how your opponent reacts.

The versions shown in this clip:

Ippon-seoi-nage
Ippon-seoi-otoshi
Morote-seoi-nage
Morote-seoi-otoshi