Hi Blakey. Depending on which mod you tried it may well have had similar functionality to your Ego C Twist, which allows you to adjust the voltage anyway. However they do usually have a bit more to them e.g. my eLeaf iStick you can vary the wattage as well as the voltage, same with the MVP and I think most can be set to auto to adjust themselves according to the coil. Plenty is the in-house expert on shiny mods, so hopefully she'll come on with advice too.
From the tank point of view, virtually all e-cigs these days come able to take the standard 510 or Ego thread, or have an adaptor which should come with the battery so don't worry about that - I currently use my 4 different tanks on standard ego type batteries, Evod twist, spinner v2 and iStick.
Many standard vv batteries are available in 1300mAh, Spinners can come up to 1600mAh, and box mods like the MVP around 2-3000mAh, the larger that number the bigger the charge & therefore longer between charges.
So it really comes down to 3 factors - just how much extra functionality you want, how long you want between charges, and the look/feel of the device. The box mods can feel surprisingly good in the hand for most people, but some don't like how they look or the look of the tank the way it sits on them. My current personal preference is the iStick, but I'm still very happy with my spinners & even the cheaper Evod twists.