What I want to know about the 'D style' carts is that it looks like vapor will only go through one side with only one side opening. But maybe it holds more juice...IDK...I don't own one. The round tips have two openings (one on either side) to allow vapor to flow through. Anybody notice any vapor difference with the D style carts?
Actually there are a few different styles of D carts too or... they simply changed some things. The original D carts flat tips I got from Eastmall had an end insert in the mouthpiece that came out rather easily - I had three come out in my mouth. And there was a thread here on someone almost choking... the thread deteriorated but it is in fact the case that they weren't my favorite for that reason and a few others. They were also thinner - which some people liked, not I. And the filler was glued to the floor of the cartridge chamber, which you had to pull apart when you washed the filler - and in a few cases that pulling would dislodge the floor from it's inset and then it was loose. You could put it back into place but sometimes there was still movement and also then juice could leak through to the actual air chamber.
As far as the airflow goes - even though it was one sided, it was a bit bigger than one of the two sided channels - that the actual area was I don't know but I think it resulted in more juice area and the airflow didn't seem to be affected.
Now one set of D styles I have are good - the end mouth piece is the same, it isn't thin, and the filler isn't glued and I'm not sure where I got it from

But I still prefer the '0' style - thick and good for holding in one's teeth, esp. if you have a 510 auto batt. And the end piece on the mouthpiece is solid on all of mine although there have been reports of that one coming off too but I think the person said they were a 'biter'...
I don't notice any vapor difference between the 0 and the D but I do notice a flavor difference between the round and the flat tip for me anyway. The flat tip give me a better seal and a better draw and more flavor.