For me, the progression was Halo Triton (eGo style), iTaste vv, ProVari Mini (with extension cap), Vamo, full-size ProVari (with another extension cap).
iTaste vv was just given to a friend who wanted to upgrade his experience. It worked fine, except for battery life. I'd kill it in less than a day of moderately heavy use. My friend is a much lighter user.
Vamo was great for a while, but the center pin became loose, and the fire button started getting flaky. Hence, to replace the Vamo, I got the full-sized ProVari.
Still have the Halo kit as a light-weight "walking around" device, and as a backup.
People place value on different factors, and that's why there's such an overwhelming variety in this market IMHO.
Factors for me on the battery side of the equation are:
1 - Battery life. Must last all day. ProVari mini with extension takes an 18490 (red AW IMR 1100 mAh). Full-size ProVari with extension cap takes an 18650 (purple Efest 2100 mAh). Either one of these devices would last me all day. Alternating them, I've got power to spare. I still carry backup batteries for each with me daily, because I'm into playing it safe like 'dat.
2 - Dependability. ProVari's have a fantastic track record. After living with my mini for a few months, dropping it several times, and never having a single problem with it, getting another was a no-brainer.
3 - Consideration of usable lifetime - Shiny new things come out every day in this market. Even my beloved ProVari's don't do variable wattage (you mean I have to do math? j/k - My academic backgound is in physics - I love math). For me, the purpose of the battery is to safely deliver clean, regulated power to whatever holds my coil, wick and liquid. That's it. Okay, a few extra points if it feels solid and it's a pleasure to hold in my hand, but those are minor factors. The ProVari does all of this extremely well, and for me it will have a usable lifetime much greater than any sealed (non-replaceable) battery unit.
I tend to believe that you generally get what you pay for. For delivering power to my coils, two ProVari's (and a microphone

).
Atomizers are an entirely separate topic. Using different atomizers will radically affect your vaping experience, much more so than different battery units will.