What has you deciding between those two mods? The iTaste VV3 maxes out at 11W and only has 800mah battery life. The Zmax Mini will use a replaceable 18350 (which is also about 800mah) and maxes at 15W. The VV3 is a thin stick; the Zmax is a short tube (and heavier than the VV3)
The iStick 30W is mid-sized between the Zmax and the VV3. Has 2200mah battery life, wider range of operation (Tootle Puffing to Cloud Chasing)....
Whenever anyone asks about "upgrading", I always like to ask questions about what form-factor is preferred, internal v. replaceable batteries, etc. I have used VV3s before - they are nice, light-weight devices. I have mods with replaceable batteries - heavier, but battery life is better.