Prefilled isn't more expensive than smoking if you keep them damp with liquid. I can get 7+ days use out of a prefilled KR8 carto. That's $2 for a week added to the cost of the juice. That's only slightly more than using a blank carto for a week. I now buy only blank KR8 cartos but when I was trying to find juice I liked, prefilled carto samplers were helpful.
Re Ego vs KR8
KR8 - more voltage (3.7v), the most powerful slim ecig, hottest stuff it should use is 2.5 ohms
Ego - less voltage (3.4v), much more charge, can use hotter LR cartos and atomizers (though sometimes items under 1.7 ohms can kill an encased mass-produced battery like Ego and others - some will only use really low ohms on mods). Some Ego-alikes are 3.7v, as are 3.7v mods, and these are other big battery alternatives in the same approximate voltage realm. if you plan to use LR stuff on Ego the 3.4v doesn't matter. If you plan to use an adapter to vape your usual KR8 cartos on it, you may notice a bit less warmth