Ok, I'll try to give this a shot, but be advised that
vaping tends to vary greatly from user to user so what works for 1 person might not work for another.
What do people recommend mostly in regards to cartos vs atomizer+carto? This depends on the user. If you are going to simple "I just want it to work and feel like a cigarette" then a good route to go is with the Volt by
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how long do each last? Most cartomizers will hold anywhere from 1-2 ml of liquid for a standard sized. They can be refilled several times without cleaning or you can extend the life a bit further by cleaning them.
how many refills would 30ml bottle give you? About 15 - 30 depending on the size of the carto.
essentially i am a pack a day smoker (20pack) People often vape differently than they smoked. Some vape a lot less as they don't feel the need to "finish" a cigarette. They can take a few vapes and be fine. Other's chainvape because they can use it when they normally couldn't smoke a cigarette. Only time and experience will dictate what your vaping habits will be.
what kind of purchasing routine, would i need to start as far as getting atomizers (if i go that route) and juice? Atomizers can lasts anywhere from a day to several months. There really is no right or wrong answer to this one. If you go with atomizers, make sure that you keep plenty of spares on hand.
(I do also realise that cartos are refillable too. so how long before those need replacing?) See above for general use. I've found with most filler type cartos that I can get 20 or more refills from them before the performance drops off.
I hope that helps. I'm sure you'll get many more suggestions.
Welcome to the forum!!