I'd encourage you to first do some retirement planning and let that guide your decision on which plan to contribute to now and how much. It's using the principle that Stephen Covey (7 Habits of Highly Effective People) used to promote "begin with the end in mind" and working backward.
So, think about when you want to retire, what that's going to look like and what you'll do, and what it'll cost to support you for the rest of your life. You may want to enlist the help of a fee-only financial planner to work with you on scoping this out.
But more directly to your question . . . two thoughts:
1. Not knowing the future, it's usually better to have saved too much than not enough, and
2. Having tax-free resources will give you options with your retirement income
When you're retired, your pension and distributions from your pre-tax retirement plan are both going to be 100% taxable. Depending on your income level, probably 85% of social security will also be taxable (under current rules).
If you've accumulated tax-free (i.e. Roth 401k) and already-taxed resources in addition to your pension and pre-tax retirement plan, you'll potentially have the option to mix-and-match from all sources in a way that may keep your overall tax burden down. It's a good idea to give yourself those options.
Hope that helps. All the best!