Some of my friends have used both and all of them said the same thing - the vest and ruck, while looking the same, are very different.
With this in mind, I have no idea what kind of weight you would use. However, I can say, without any doubt, that if done for conditioning reasons, most people use way too much weight. So don't try to make this into a challenge. I just finished the conditioning section of my new book and I repeat, many times, the conditioning mantra that most people and trendy shit-programs forget:
"Training is not testing."
Let that be your guide.