Here is a criteria not often discussed: Which one would be easy to switch from if you ever wanted to do that?
The answer is that Vanguard will be the easiest to switch from. The reason is that Vanguard creates simple portfolios without complexity, so that they are easy to understand, easy to implement, easy to be tax-efficient.
Betterment can make it hard to switch. They will put you into too many different ETFs, they make understanding the 1099s harder, they make it harder to unwind a portfolio with all the attention one has to pay to cost basis.
Of course, if you don't have any taxable account assets and your assets are all in IRAs, then no problem with Betterment.
Also, if you like complicated, then Betterment is for you.
As far as performance goes though, there will be very little difference between the two provided that the stock:bond ratios are the same.
I'm a do-it-yourselfer, so Betterment offers me nothing that I cannot do myself. And Vanguard offers me nothing that I cannot do myself as well.