ETFs are in general more tax-efficient because they don't have to distribute capital gains unlike mutual funds. In additional to dividend distributions, you realize your capital gains only on the sale of the ETF.
Vanguard's funds are an exception. This is their VTSAX (Total Stock Market Index Fund) performance page:
And this is their VTI (ETF version of the fund above) performance page:
We see the after tax difference is either 0.01% higher for ETFs, or tied, and one time the fund was higher. However, when we consider that you have to deal with the spread when buying/selling ETFs, and that the spread typically gives you a penalty that's
"typically in the 0.01% - 0.1% range (varying with fund size and popularity)" both when you buy and when you sell, we see that there is no tax advantage for ETFs when using Vanguard. Bogleheads explains that ETFs are typically more tax efficient, but "The exception is Vanguard's dual-share fund structure, which allows their index funds to be just as tax-efficient as ETFs."
http://www.bogleheads.org/wiki/ETFs_vs_mutual_funds#Tax_efficiencyPersonally, I choose the fund over the ETF, as the expense ratio is the same for me, I don't want to deal with the spread of an ETF (it makes ETFs more expensive, even for the same ER), I like buying/selling in fractional shares (you can only buy the ETFs in whole amounts, so there will always be some money sitting on the side), and I like to use Automatic Investing, which is not available for ETFs.
To highlight the expense difference, Vanguard made a nice comparison tool:
https://personal.vanguard.com/us/faces/JSP/Funds/Tools/FundsToolsEtfCostPurchInfoContent.jspWhich shows that the fund ended up being a tiny bit cheaper based on the numbers I put in:
So it isn't a big difference, but I didn't see any advantage to ETFs anyway, so going with the fund was an easy choice.