I'm not a really big smoker
Atomisers definately last longer but are somewhat less portable than other options.
The egoT (tank) is an atomiser with a larger capacity for liquid without filling and could work for you as a more casual smoker.
How many cigs a day are we talking here? The volt could be the way to go if you're just after a cig looking thing you don't need to use too often if you're ok to use cartomisers (volt cartos I think are one of the best and cheap).
However if you want an atomiser type unit, a joye 510 will be about the same size and you can use a 510 tank atomiser or a normal atomiser. I would probably reccommend a 510T atomiser kit based off your needs when I wouldn't normally. An Ego kit could be too big for you. Just depends on how many you go through a day or not.
With the ego or riva you have the choice: you can use cartos or atomisers so it's the cheapest thing to buy once you have the kit, just an atomiser or pack of reuseable cartomisers every now and then and some eliquid to put in them. Couldn't be easier or cheaper if you want something that'll last a while with low maintenance.
The joye 510 is simply a smaller version of the ego or riva. You have a choice stil: atomisers or cartomisers or a tank system. So if you want smaller size and don't mind a smaller battery life it could be for you. Very cheap.
I'm sure the american guys could point you in the right direction for vendors. I'm sure you could get a single 510 atomiser from somewhere really cheap. They can last a few months with heavy use where cartos are a few weeks.