My gut reaction is a 401k, but there are some things to consider.

First, does your plan allow a Roth 401k deduction type? If so, you could still get the Roth feature within your plan without opening a separate IRA outside of it to keep your retirement funds together.

Second, there is a limit to what you can contribute to a Roth IRA that may not apply to your 401k, so depending on what 6% of your salary is, you may not be able to put that much into an IRA which will lead you back to your plan for the extra contribution.

Third, whether you use a Roth or not for the 6% should be based on your tax situation and you prediction of the future. If you think that your tax rate will be higher at retirement you may want to pursue that Roth option.

Fourth, what investments can you select from in your plan? If you are happy with the choices this is a non-issue. If you are very limited you may wish to put the 6% in another product like a Roth IRA.

To keep things simple I would tell you to put 10% into your plan unless one of the things I pointed out above compels you to get the money out of the plan. A hybrid option would be to put $5000 into a Roth IRA and the rest (up to your 10%) into the 401k.

To decide which to invest, it is very difficult. If you expect your income will be gratly increase in your retirement years, you should invest in Roth. However, no one knows the future. I would spilt the investment in traditional and roth. 5/5, or 4/6.

To decide which to invest, it is very difficult. If you expect your income will be gratly increase in your retirement years, you should invest in Roth. However, no one knows the future. I would spilt the investment in traditional and roth. 5/5, or 4/6.

I believe you are better off putting that extra six percent into your Roth IRA instead of your 401k for many reasons. With your 401k you probably only have about twenty investment options, with your Roth IRA you have probably up to one thousand investment options. With your 401k they sell your company a package of investments that have set fees. Oftentimes these fees are very high up to five percent or more. With your Roth IRA you can buy funds that are low cost or no load or even index funds which have very low fees. So basically you are able to control costs a lot better with your Roth IRA. Now when you go to your 401k person at work and ask for advice with your portfolio allocation they will tell you well we can't really give advice the company doesn't allow that. But, when you call the brokerage that you have your Roth IRA at you will probably have an advisor assigned to you who will answer all your questions and help you out anytime you call.
So basically the Roth beats the 401k in my opinion in the following areas. You can control costs. You have many different investing choices versus just the ones the company picked out. You can get good advice when you need it for the little questions you have when you are just learning and also for the bigger questions you have down the road when that little six percent grows to be a lot of money. The only time I believe the 401k beats the Roth is that your company gives you free money to fund it. Also the fact that you can put away such a large amount yearly.