Both ways work, and YMMV as to the number of fills you will get, but the performance is roughly equivalent, I have found. For some reason, my Premium Smilin Menthol prefilled coolcarts go forever. Blank Coolcarts filled with Wow Peppermint also seem to function well with more refills. I am not big on cleaning/boiling cartos, though, so someone else will need to chime in about performance after cleaning.
I enjoy getting prefilled cartos with matching
juice because they are then labeled with the juice name and strength. Some people don't like the color that the prefilled cartomizer comes in, so will order juice and empty cartos.
Another reason to order separately is that the prefilled carto is not your preferred type of carto. (I like Coolcarts, so I get WoW juice, and put into cool carts. I'm sure there are folks who do the opposite as well.)
When I am first testing flavors, I tend to buy the prefilled, and order the juice later if I like it. Sample packs are awesome for this, too!
The sale going on now through Oct. 29 is for 21% off, and the coupon code is "Customer".