Dear Politically:
Hi, and welcome!
Ok, to answer your question - the structure of an atomizer stand-alone is quite different from the atomizer portion of a cartomizer. And they are more durable for the most part.
A single atomizer can with care last 6 months or more. Cartomizers do not tend to last more than 5 to 10 refills, then either the draw gets hard or there is a loss of vapor production or they begin to taste bad.
Ok, let's say you pay $8.95 for an atomizer and it does last six months (true sometimes they only last two weeks and sometimes two months. User variations and manufacturing tolerances account for that.) Then if you
vape 4 carts per day (in my experience about equal to two cartomizers per day) you will use that one atomizer 720 times.
If you use two cartomizers per day, and each cartomizer lasts for 10 refills and there are 5 in a pack, one pack of cartomizers will last for 500 refills.
It actually averages out cost wise pretty closely. The real reason for choosing one over the other is which tastes best to you, imho. And there are many different opinions on that subject.
I've only had one atty fail since I began, and that was four months ago. I've two attys still going after 4 months. And cartridges are - again imho - much easier to clean, and just as easy to refill.