The MVP 2.0 Shine will have a much lower output maximum, at 5 volts or 11 watts respectively. They are proven to be one of the most durable mods out there though, and have the ability to charge other electronic devices. The 510 threading is a little weak and can strip if you aren't careful. The center connection pin also has a tendency to sink if you screw your atomizer on too tightly again and again.
The eLeaf iStick will have a higher output maximum, at 5.5*(6) volts or 20 watts respectively with the right ohm coil. The 510 threads are VERY weak and you should put an adapter on it to accommodate as soon as possible to increase the lifespan of the iStick threads. The center pin is also suspect, and can sink if you aren't careful screwing on your atomizers too tightly. The power output at lower settings is all over the place, so the display will not be correct when you set it for lower numbers - it will actually be outputting at much higher values (3 volts setting will actually be putting out closer to 4.1 volts).
Both mods are well received and well liked. The iStick is more compact and has a slightly shorter battery life at 2200 mah compared to the MVP Shine's 2600 mah. If you require more power, the iStick is the right choice. If you don't require more power, the MVP is the right choice. If you prefer smaller mods, the iStick is the right choice. If you want longer battery life, the MVP is the right choice, etc, etc, etc.