Pick One And Move On
AGG and BND are the Coke and Pepsi of bond ETFs — they're so similar that picking between them is a coin flip. Both track the US investment-grade bond market, meaning you're getting a mix of Treasuries, corporate bonds, and mortgage-backed securities. AGG is from iShares (BlackRock) and tracks the Bloomberg US Aggregate Bond Index, while BND is Vanguard's version tracking a nearly identical index.
Performance-wise, they move almost identically. This is one of those decisions where the answer is "just pick one and move on with your life."