Personally I'd want those loans gone ASAP. It was one of the first financial moves DH and I made as a married couple- knocked out his student loans. It was only $12K, but it felt really good to be rid of it.
So if you go taxable, yes, the gains will be taxed at 0% when you harvest them... but what will the tax be on the gains? Probably not close to what you are putting in. As such, I would consider putting more into the 401K. Yeah, it's only 15% you are paying, but why not pay nothing? Also, this lowers your AGI, and if you can get it below $60K, you qualify for the Credit for Qualified Retirement Savings Contribution. (Form 8880) It's not huge, but it's still fun to get tax credits.
Let's say you have an amazing 20% return on your taxable investment. You put $1K in a taxable account, paying $150 in taxes on that income. You have a nice year and end up with $200 in capital gains. You harvest those gains of $200, basically getting away with not paying taxes on $200, a savings of $30. But you paid taxes on $1,000 for that privilege. Had you put the $1K in the 401K, you would have saved the $150 in taxes. Seems like the 401K is the way to go over taxable.
I would still prioritize the Roths over the 401K. It's just a great, flexible investment vehicle, with no taxes on the gains ever, not just while you stay in a low tax bracket. Then max out the 401K. After that, you can cry and say "I have so much money... I have to use taxable accounts!"
Congrats on getting married!