See if your employer does a 401k match first. Like, if you contribute $2000, they match some percentage, commonly 25-50%. So instead of $2000, you get $2500-$3000 in your 401k account. The employer puts a cap on how much they match, however. 401k contributions are not taxed now (if you are at the bottom of a tax bracket, you might just drop to the next lower one!), but rather when you withdraw the money.

Roth IRAs are money that you've earned that's already been taxed, so it's not taxed when you tap into it during retirement. These are best for people whose employers do not offer 401k plans with matching.

You will also want to consider what tax bracket you are in now. If it is high, you may be better off with the pretax 401k. The withdrawals you make in retirement count as ordinary income and may be in a far less tax bracket than you are now. In the example above while the 80,000 would be tax free, the initial investment of $10,000 would not be equal due to taxes.