My work uses Principal for 401k as well. You may have a different plan or investment options than me, but FWIW...
I went with the large cap S&P 500 index with 100% of my contributions. The ER for that one is .31%, a decent amount lower than .51%. In my plan there are 30-something options. I'm about 90% sure that the large cap index is the lowest fee, but it's also not easy to compare the fund choices. You have to dig into each one to see the ER.
If you're curious about the performance...I only became eligible in February of this year and my first contribution was in mid-March. So far I've contributed 3500 and the balance is at 3663, a gain of 9.88%. This doesn't include any employer matching, as mine does all of the matching at the end of the year. Yes, my work plan really sucks. 10% is pretty good, but the market is doing well right now. For a while there in the spring, the market was doing poorly, so I had less than I had contributed! But, Mustachians don't care about such things. You contribute every paycheck and don't think about the market.