Since you don't fly DL much, which would you like silver status for the one flight a year you might take using miles with your family? And if you do get UG'd, would you even use it, since you would be separated from your family?

Based on what you said, I see the silver status with DL being almost totally worthless to you. Make your decision on other factors.

If you transferred 50K, in reality it costs you 30K. Do that three times and 150K really costs you only 90K. So from a pure miles/numbers sense the 50K transfer with the 20K bonus reward sounds better to me.

How many DL miles do you have in each account?