One of the main differences between the sake and soju is that sake will make use of rice so that the drink itself will be a bit starchy. You can expect that its taste will also be a little bit more neutral especially when you would compare it to soju.
The soju will also be made out of different starches but what makes it different is that the starches are meant to affect its taste which can make it sweeter than what you may normally expect. Soju is considered to be sweet while the sake is known to be dry.
The way that they are made is also different. The sake is usually fermented. This can be similar to how beer is made. The soju is known to be distilled and it will be similar to vodka.