Bond ETFs are just another way to buy a bond mutual fund. An ETF lets you trade mutual fund shares the way you trade stocks, in small share-size increments. The content of this answer applies equally to both stock and bond funds.
Feature Mutual Fund ETF
__________ ____________ _______
Purchasing time End of day Midday
Holding Fees (Expense Ratios) Generally higher Generally lower
Purchase/redemption fees Zero or significant Zero or same as stock*
Minimum for initial buy Generally higher Price of a share
Additional buy increments Any amount Whole shares
Dividend reinvestment Generally yes Generally yes
Value = underlying holdings Almost always A little drift
* including bid/ask spread, which does not apply to mutual funds
If you are intending to buy and hold these securities, your main concerns should be purchase fees and expense ratios. Different brokerages will charge you different amounts to purchase these securities. Some brokerages have their own mutual funds for which they charge no trading fees, but they charge trading fees for ETFs. Brokerage A will let you buy Brokerage A's mutual funds for no trading fee but will charge a fee if you purchase Brokerage B's mutual fund in your Brokerage A account.
Some brokerages have multiple classes of the same mutual fund. For example, Vanguard for many of its mutual funds has an Investor class (minimum $3,000 initial investment), Admiral class (minimum $10,000 initial investment), and an ETF (share price as initial investment). Investor class has the highest expense ratio (ER). Admiral class and the ETF generally have much lower ER, usually the same number. For example, Vanguard's Total Bond Market Index mutual fund has Investor class (symbol VBMFX) with 0.16% ER, Admiral (symbol VBTLX) with 0.06% ER, and ETF (symbol BND) with 0.06% ER (same as Admiral). See Vanguard ETF/mutual fund comparison page. Note that you can initially buy Investor class shares with Vanguard and Vanguard will automatically convert them to the lower-ER Admiral class shares when your investment has grown to the Admiral threshold.
Choosing your broker and your funds may end up being more important than choosing the form of mutual fund versus ETF. Some brokers charge very high purchase/redemption fees for mutual funds. Many brokers have no ETFs that they will trade for free. Between funds, index funds are passively managed and are just designed to track a certain index; they have lower ERs. Actively managed funds are run by managers who try to beat the market; they have higher ERs and tend to actually fall below the performance of index funds, a double whammy.
See also Vanguard's explanation of mutual funds vs. ETFs at Vanguard.
See also Investopedia's explanation of mutual funds vs. ETFs in general.