Oh, I have a similar question.
The most "index" like fund I see on my company's list is "Black Rock S&P 500" BSPIX. The expense ratio is .24. That is better than the rest of the options, but doesn't exactly seem "good." The company doesn't match, but makes "safe harbor" contributions and sometimes voluntary contributions. It seems to me that the only benefit of using the 401K as opposed to a lower expense fund is the tax benefits, which increase my contribution by 75%. (i.e. lower paycheck $100, contribute $175 to the fund for example, 200/350, 300/525 and so on) Is the tax benefit enough to make up for the difference between a .24 expense ratio and a .15 expense ratio I could probably get someplace else? I think so, but I'm not sure how I'd figure it out.