March 15, 2012:   What is the best exchange traded fund (ETF) for bonds that attempts to track the Barclays Capital U.S. Aggregate Bond Index? Here are the four choices I want to consider.  Send me an email if you know of any others with the same characteristics including low expense ratios.
 
SCHZ  = Schwab U.S. Aggregate Bond ETF

"The ETF seeks investment results that track, as closely as possible, before fees and expenses, the total return of the Barclays Capital U.S. Aggregate Bond Index. "

BND:  = Vanguard Total Bond Market ETF

"The fund employs a “passive management”or indexing—investment approach designed to track the performance of the Barclays Capital U.S. Aggregate Float Adjusted Bond Index."

AGG: = iShares Barclays Aggregate Bond

The iShares Barclays Aggregate Bond Fund seeks investment results that correspond generally to the price and yield performance, before fees and expenses, of the total United States investment grade bond market as defined by the Barclays Capital U.S. Aggregate Bond Index.

LAG  = SPDR Barclays Capital Aggregate Bond ETF

The SPDR® Barclays Capital Aggregate Bond ETF seeks to provide investment results that, before fees and expenses, correspond generally to the price and yield performance the Barclays Capital U.S. Aggregate Index (ticker: LBUSTRUU).

First look at annual expenses.  Schwab is the leader with the lowest as of today.

Fund symbol & expense ratio comparison
Schwab	Vanguard	SPDRs	iShares
SCHZ 0.10%	BND 0.11%	LAG 0.17%	AGG 0.22%