There are only a couple of differences between a V2 and a V3.
A V2 mechbox is known to be weaker in the front of the mechbox, but many companies (G&G, for example) have reinforced gearboxes that usually eliminate that issue. Also, the motor is not mounted to the actual mechbox - making alignment and adjustment more difficult (and reducing overall reliability).
V3 mechboxes are very durable and reliable. It has a solid design that can take loads of abuse. Also, the motor is mounted to the mechbox - making motor alignment very precise (adding to reliability)