I'm gonna go the other way and say to upgrade your tank, first. If you're not satisfied with the vape you're getting right now, you won't notice much difference with the iStick if you're still using that same EVOD tank. You'll be able to turn the power up a little higher, but that's it, and the tank might not be able to handle that extra power very well, anyway. What is it you're looking to achieve, by upgrading? Mind, vapor comes from the tank, not the device that's powering it. If you really want to "upgrade" your experience, you'll probably want a new tank first, regardless. Then, however, the trouble is, that in order to power a better tank, you're going to need a new device, also. The Nautilus Mini will work, technically, on your eGo-style battery, but it would work better with a higher-powered device like the iStick.
...is your battery VV already? My google-fu suggests that it might be. Is the bottom end-cap like a knob that you can twist back and forth to turn the voltage up and down? If yes, then you've basically got a Vision Spinner, already. "Vision" is the brand name, and "Spinner" is the model, but all told it's just the same as any other variable-voltage eGo-style battery like the EVOD VV battery from Kanger or the eGo Twist from Joyetech. In fact, the one you have right now might well be a re-branded eGo Twist from the same factory, just with a different logo on it. Or it might not, you never know. But for just a few dollars more (or pounds, whatever) you can get the iStick which will have even more battery life, in a smaller form-factor, and you'll be able to turn the power up higher (and have more control over fine-tuning the power exactly to your liking) than on any spinner type eGo-style battery. But if it's really down to one or the other, I suggest you get a better tank, first, and hold off on upgrading the device until later. If you do the other way around, you might be disappointed when your EVOD tank gives basically the same experience as you're getting already.
EDIT: After a little more googling, I see that it's not a "twist" or "spinner" style VV battery, but you do have three different voltage settings available by clicking the button three times to cycle through them. I still don't think you'll get much improvement by switching to a spinner. My advice hasn't changed: get a new tank first, the Nautilus Mini is a decent option, and if/when you decide to upgrade the device also, the iStick 30 is also a good choice. Don't waste your time and money getting a spinner, as it really won't be much of an upgrade. You'll be looking for the next best thing soon enough, anyway, might as well have something decent like the iStick in the meantime.
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