Yeah, it really comes down to what's important to you:

Would you rather get up early, spend a half day at IAD, but ensure you are home Sunday?

Or would you rather get up at a better time, have a really tight connection at IAD, and potentially miss your flight forcing you to stay in Northern Virginia through Monday (or possibly even Tuesday)?

If you prefer the first, then you take the earlier LHR->IAD flight. If you don't mind the second, then stick with your original plan