Well besides the overall size and shape differences between the v3s, MVP, and iStick the one thing not to forget is the wattage range as well as the two Innokin products offer the separate voltage control. On the iStick as well as many other newer APVs that say VW the voltage moves along with the wattage and vice versa with seeing the wattage move while in the voltage mode.
On those two you also have the 6w-11w range while the iStick runs right in there about 4.7w right upto 20w and if you went over the 11w max on the others that would account for the warmer vape to begin with. Here I run now only but did have four eGo-V V3s until two let go as well as four V4s in use having received another four to be set aside as spares until either one of the four v3s or one of the other eGo-V batteries is the next to go.
Each one has the exact same tank but different color as well as 2ohm coils in each of those Smok Tumbler tanks. When coil's wick goes, the battery wears down, or when the wattage is up too high per flavor you'll get a nasty one that will feel like the tip on a soldering iron! If the coil is still good once the battery is recharged or the wattage is lowered you then notice the immediate improvement. The same goes for the Nautilus and Atlantis tanks only at much higher wattages with fruit and berry type blends taking on the higher wattage while a menthol tobacco flavor comes in at 20w-25w compared to the other 30w-50w depending on which flavor and where it seems to max out.
When going from MVP to Vamo to eGo VW batteries with the same tanks one flavor would vape best on a Vamo while others on the MVP or VTR while they would suffer for some reason on the Smok ACEs that had the same battery as one of the Vamos. And this is why I went for the Eleaf LCD Ohm and Power meter I can place in between tank and battery, mod to get the real time information once the resistance factor is accounted for. The 7w or 10w on one doesn't necessarily mean the same is being seen on the next if the amperage is off a bit as another item.