Never played with the DE250. Im ordering one from Wayne P. in the near future (for me that might be a year out ) to build something very similar to his Pi3 speakers, I already have the TD12S drivers now unused.
Honestly I doubt anyone can go wrong with the DE250. Mark and John have their conclusions but we also know that many others like Wayne P., Dr. Geddes have their conclusions too. Wayne has been around a long time and has some experience, he once told me that the H290/DE250 + TD12S (3 Pi upgraded design) is one of the best speakers he has heard ever. Of course , again that is also subjective since I think the TD12M is a better driver then the TD12S.
Sometimes we have to remove/ignore the subjectivity to realize that the DE250 and the BMS series drivers are both damn good drivers in the right design.
For the SEOS build design we have to remember that its throat angle matches the DE250 so I will say that the DE250 should be the driver of choice for that design. I would recommend reading some of Augerpro's thoughts on the QSC and B&C drivers, vs BMS or even higher end Beyma. Look for his "No Quarter" build/discussions thread.
Its 100% okay for Mark Seaton to choose another driver because he is selling the entire kit. He isnt here to build a DIY XO for the rest of us (even if we want him too )