In terms of build quality they are made almost exactly the same and weigh in at a hefty near 5 pounds, the only differences is the inner barrel and magazines to accommodate the different sized ammo. The BB version has a 25 round single stack magazine that does not have a follower to stop the bolt from continuing to operate even after the magazine is empty while the 6mm Airsoft version has a double stack magazine that holds 38 rounds and does have a follower to stop the bolt which can save you some wasted CO2.
The Airsoft version Mini UZI is distributed by KWC while the 4.5mm BB version is distributed by Cybergun, you will find the UZI and IWI licensing on the Cybergun 4.5mm BB version but not on the KWC 6mm Airsoft version.
Out of the box the KWC Airsoft Mini UZI is ready to go in both semi auto and full auto while the 4.5mm steel BB version will only shoot in semi auto mode, it can however be easily modded to shoot in full auto as well.
Make sure to check out my other video reviews for more information on the Mini UZI: