Every ported sub I've heard I always heard port chuffing when pushed to the upper limits, and always heard sealed are more accurate, but not as much SPL, plus if I were to build, I'd have no idea about port tuning.
The boxes for the ported subs that you heard were not well designed. I would suggest that you get the BassBox 6 Pro software and if you follow their recommendation for minimum port area, you shouldn't have any port chuffing problem.
Personally, I have been using two 4" port tubes for 15 inch subs and I've never heard any port noise on the enclosures I built. Of course, if you intend to drive your subs at extremely loud levels sometimes beyond their power ratings, there may be some chuffing, but if you build cabinets by following recommendation with a serious speaker building software and operate them at reasonable home SPL, that shouldn't be a concern. If you want to be really safe and can build a bigger enclosure, you could use two 6" tubes with one UM18 for instance. That would require long tubes that take more space within the box with resulting increased box dimension. Don't forget to use adequate bracing and plenty of glue for good results.
Sealed subs are not more accurate than a properly designed ported sub. As a matter of fact, sealed subs are not as efficient as the ported type and they also have more distortion at high levels. In addition, they can't reach the low frequencies which the ported one will unless they are heavily EQd with the resultant distortion. If you have the room space, go for the ported design with subs that are designed to be used in a ported box.
