I prefer simple LR 2 ohm cartomizers over the various tank setups. I can carry around my favorite dozen flavors in a dozen carts that easily fit inside the space of a pack of smokes. Can't do that with tanks.
And don't you just hate it when a pocket full of tanks begin to leak in your pocket and saturate your pants with nicotine and you have to endure the barfy trip home to change cloths without clothing covering your lower half? LOL. No more tanks for me.
Refiling carts every evening has become a bit of a pain but a ritual with me that is worth it to be able to bring along my dozen favorite flavors wherever I go.
How much they hold is easy to determine, just count the drops it takes to fill a new one. About 20 drops should be around 1 ml. I fill them until the fiber on the top begin to show a slight shine, meaning it won't absorb any more with the next drop of liquid. Spin between your hands to make sure it's fully saturated (if not the fiber at the top will lose that filled shine).
Look down the center of the open end until you see the small hole at the other end begin to close up slightly, that means it is full. If it completely closes, that means you've over filled it. if any liquid remains on the surface of the fibers, then you've over-filled it. Once filled, blow lightly through the threaded end to simply clear it, and while hold the cart upside down screw onto a battery and take a couple long warm-hot hits while continuing to hold it upside down and then it should be ready to go and without leaking due to any excess added. If you over filled it, you will have to hit on it for a while otherwise it might leak down into your battery contact and you'll get a gurgle sound when drawing on it and you'll have to clean it out.
When using liquids with higher VG content, spin the cart in your hand to make it more quickly absorb the drops you put in. Let the drops begin to run down the side of the cart and into the fiber and then spin. If you use a needle tipped refill bottle, you can poke lightly at the fiber top to also cause it to absorb more quickly.
Once you learn to fill the carts, it become easy but still remains a choir I must endure every evening in preparation for tomorrow at work. It does take me about 15 minutes to refill what I use every day. Not as easy as tanks, but I've given up on tanks forever because they crack and leak, not always, but it has happened enough times to cause me to give up on them.