First: 401k max is $18000. IRA max is $5500. Roth or traditional doesn't matter. I think you get that but just so it's clear :)
Mad Fientist has a good post on this topic:
http://www.madfientist.com/traditional-ira-vs-roth-ira/Basically, because Roth anything is post-tax money and traditional is pre-tax, you can invest more and thus get more return from a traditional 401k/IRA. So you're right on that. The issue is then avoiding paying a buttload of taxes on your future withdrawals. Your options:
1. Use a traditional 401k and IRA. When you quit your job, roll your t. 401k into your t-IRA. You wouldn't want to convert a traditional 401k into Roth IRA because you would pay taxes on all that money in the year you quit your job and roll it over. Mad Fientist argues you should use traditional everything during your working years since you can convert a traditional IRA to a Roth IRA in early retirement, little by little as the years pass, by staying within the standard tax deductions, since t-IRA deductions count as standard income. You could pay little to no taxes this way and get the best of both worlds. You could also pay about the same or more tax depending on where you get income during RE, since this money is being converted to a Roth for later use in most cases, and you can't always predict the future accurately. Some people think the government might change the rules eventually and tax growth on Roth IRAs/401ks, and this method also requires keeping a close watch on everything year by year after RE. So: trade-offs.
2. Use Roth everything. After tax, simpler. MF says this keeps growth lower, but if you're in a lower tax bracket now than you think you might be after FIRE for some reason, it could be a good idea.
3. Mix things up to hedge your bets! Usually, if you buy into the MF way of thinking, there's a clear path for you each year depending on your salary and tax bracket, but it really depends on what you're comfortable with and what your future plans are (as well as your current and future tax brackets).
There are also mandatory withdrawals once you hit 70, I think, with t-IRAS, and those don't exist for Roth. So your future self might thank you for using a Roth, but again, the Roth conversion is an option.
Note that Roth conversion is different from the Roth pipeline, which is a way to access Roth IRA funds without paying the 10% penalty before turning 59.5 years old.
Good luck Cheddar Bob, and please, someone correct me if I got something wrong - it's a complicated topic.