I want to caution you all by saying that I am not an investment expert! I would advise that you seek the counsel of a financial investment adviser when choosing your investments.
However, the average return I used was based on the overall stock market of 10% SEE LINK (average over the past 20+ years). Most 401ks or IRAs are mutual funds which are based on a diversified grouping of stocks. I wouldn’t look to the stock market for short term returns. Retirement investing is for the long term.
Typically, a 401k (403b) should be more beneficial because usually a 401k offered through an employer has a contribution match. Therefore to follow my example...If an individual makes $52,500 annual salary and contributes 8% of their income per month ($350), and receives a 75% company match on the first 8% of their salary contributed to the 401k; their $350 "car payment" would actually be $612.50 a month. Therefore, this can only benefit them in the long run to the tune of about $3,873,499 (40 years) or $2,325,441 (35 years). This also doesn't include increases in income over the years which will also subsequently increase the contribution and matching amount.
Bottom line, It’s all goodness...
As for an IRA, usually these do not include a match, and they come with their own annual contribution limitation, however one can choose to go with Traditional or Roth IRA's that provide different tax advantages. In many cases individuals can contribute to both a company 401k and an IRA.
Hope this helps...