The main difference between the two is low bridge atomizer (RN4075) and high bridge (DSE901), as shown in this photo:
Low bridge reputedly wicks better with cartridges. [I wouldn't know; I always drip.]
You can get RN4075 attys from
VaporKings and
Puresmoker ... and probably other suppliers, but those were the ones I found when I last researched this a couple months ago. VaporKings currently has them on sale for $6.