As a general rule, it is important to pay attention to opportunity cost. You can only use each dollar once, so using it somewhere means you can't use it somewhere else. So don't hyper-focus on what you will "save" with one choice without also looking at what that choice will cost you.
In your particular situation, you have two choices for that dollar. Option 1 is to put it toward your student loans. What does that get you? You avoid paying 6.5% interest. That is a good thing. However, as interest on SLs is tax-deductible, you don't actually "save" the full 6.5%, probably more like 5%-ish.
Option 2 is to put it toward your 401(k). What does that get you? First, an immediate tax deduction. Second, some of those dollars will get you an employer match, which is an automatic, guaranteed return on your investment somewhere between 25%-100%. Third, every single one of those dollars will grow tax-free for the next several decades, generating more dollars, which in turn can grow tax-free and generate more dollars. But here's the other thing: Option 2 is so awesome that the government has to limit it, or else all the billionaires would never pay taxes. You can only put $19K in your 401(k) every year, and if you don't use all of that in a particular year, you can't "make it up" by investing more in later years. So every dollar that you don't use to max out your 401(k) represents decades of tax-deferred growth you are giving up.
Of course, you are going one step beyond that and considering actually taking money out of your 401(k) to pay your loans. That has one more major cost that just diverting money to paying the loan does not: you pay a 10% penalty for taking your money out early. Think of it as "undoing" your past decisions, with a kicker: in 2018, say, you put $18,500 into your 401(k), so you got to knock $18,500 off your income that year, and that money is growing. Now you are undoing that choice. So first, you are voluntarily forfeiting forever decades of tax-deferred growth on $18,500 of your 2018 earnings. Second, you get to pay extra taxes in 2019 for taking that money out -- every penny that you take out in 2019 counts as "income" on your 2019 taxes. And third, because you have to pay taxes on what you take out, you are going to need to take out more than the $16K you need to pay off the loan to end up with $16K in hand. And finally, the real kicker: you get to pay an extra 10% penalty for an early withdrawal to boot. So all together, you'll probably have to withdraw $20K or so to pay off $16K in loans, thus forfeiting forever the right to tax-deferred growth for all of your 2018 contributions and some of your 2017 contributions to boot. So, basically, in return for forfeiting your right to grow many many additional dollars over the next several decades, you get to pay taxes and penalties now, and give up the student loan deduction on your future taxes.
IMO taking money out of a 401(k) for anything is a real last-ditch effort and should be deployed only when all other efforts to cut expenses are insufficient and the loan rate is usurious. That's not you, so don't do it. In fact, I would not even cut back on the 401(k) contributions. Look for other ways to cut expenses to get those loans paid off. For bigger-picture issues and analysis, check out the investment order sticky -- it will give you a good framework for walking through these types of decisions.
