I'm sure it's been covered to death, but can somebody explain (or offer a link to a good explanation) the pro's & con's of each.
Also, looking on the Vanguard ETF list I see the MGK yields are higher than the VTI, so would it be a better choice?
In a taxable account the best choice is Vanguard Admiral Shares mutual funds which have an ETF version available, because the ones with an ETF version are less likely to distribute capital gains which force you to pay taxes. For all other fund companies ETF's are better in taxable accounts for the same reason, just be sure to keep the expense ratio down and choose a fund that trades easily.
In tax sheltered accounts there is no advantage, choose either mutual funds of ETF's. I suggest that mutual funds are the better choice because they do not require personal interaction, you can set them to invest automatically. Generally the less you interact with investments the better you will be. Also, I expect mutual funds may have fractionally lower lifetime expenses than an identical ETF. Not that big a deal in accumulation because you buy earlier in the day and are more likely to catch a dip, but in retirement mutual funds sell later in the day when prices should be, on average, higher.
VTI has a higher yield than MGK. Do not choose MGK. VTI is the better option.