I own and use both. Had 8x 4-gallon Rotopax at one point, sold half of them off when I sold my Tacoma because I mainly got them to use with an AllPro bed rack. The Rotopax are super handy for folks who have limited auxiliary fuel carrying capabilities, namely because you can simply strap them down flat on a roof rack. We've done that a few times, on a number of different vehicles that lack rear bumper's with fuel carriers, and never had a problem with leakage (they are advertised that way). Like others said, you have to crank those tops down tight.
I like that Rotopax have a larger area with lots of handles, making them easier to pour solo. That said the CARB fillers they provided for me when I bought them back in 2014-15 are really, really slow, it takes us forever to empty a Rotopax. The Wavian/NATO style tanks I have, with provided CARB fillers, seem to pour faster, although we've never done a timed test. We also mix in non-CARB fillers for the NATO tanks when out of state, so my impressions may be biased.
That said, if I was advising someone on fuel containers, it'd start with where they're planning on storing them, how you'll be storing them, and how often you want to access them.
Rotopax ratchet-strapped to a roof rack can take a lot of time to access and re-secure, I think it's a great solution for the occasional long-distance trip, but not something to be relied upon regularly (it'll get old fast). Rotopax mounted with with Rotopax-branded mounts are much easier, it's a really slick system. NATO cans on the roof in a proper mount, or on a bumper mount, are likewise easy to access. NATO cans are arguably easier simply from the fact that more people are familiar with them. They also stand upright on their own at the gas station, making them easier to fill.
I'd take a big picture look at the overall vehicle build, how often and where the extra fuel will be needed, and then choose. I don't think there's a bad choice here, so long as we're considering a quality NATO-style can.