NDR is a great race and I wouldn't consider it tougher than a marathon yes there are some hilly sections but if you're sensible and don't run too hard on the hills, then you should be fine. The scenery is fantastic and it's a really friendly race.
I would say training on the south downs would compare.
There are so many reasons why Thanet is special - Firstly the organization is very slick, informative race packs, plenty of water provided, great support regardless of where you are in the field, nice trophies (if your lucky), decent finishers medal with T-shirt. great value for money. The atmosphere is brilliant and I would chose a small marathon like this over a large city one every time.
The course I would describe as challenging but thats only because the weather is normally very hot or very windy and if your really unlucky you'll get both - to me this marathon isn't about fast times or PBs it's about soaking up the great atmosphere and enjoying the lovely coastal scenery - it's by no means the hardest marathon I've run but when you cross that line you'll feel a great sense of achievment and that's what makes Thanet great.
This race is also very personal for me as I lost my father in 2006 two weeks before Thanet which I won (the ladies race)...I've never won anything before and I was really chuffed with my win considering the circumstances.