If you're worried about fees, Betterment is a mistake.
Fees in 401Ks vary - some plans have very good investment options, better than you can get on your own. Others have much worse investment options.
If you do rollover, either Vanguard or Fidelity offer very good, low cost index funds - either choice is a good one. I personally would go with Vanguard, but Fidelity is great too, if you choose there low-cost funds.
If you think you'll want / need to do a backdoor Roth IRA any time soon, you might leave the money in the 401K - tIRA money gets in the way of doing that.