Hopefully I can help.
I have an Alien, a GX2/4, and the Captain(plus too many others to get into).
I have owned the G-Priv. Its a nice built device.
Pros:
The size is not too bulky or heavy.
Great fire bar
The Modes(Min, Soft, Norm, Hard, and Max)
The battery door has very strong magnets on it and you can now purchase different colored doors.
Cons:
The extra little button just above the fire bar can get touched by accident.
The screen didn't seem very responsive for being a touch screen.
The case it comes with is kind of pointless(it only covers the corners and slides around easliy)
Updating the firmware can be a pain but as long as you DON'T follow Smok's instructions it's easier. (Release fire bar at step 3 not 5)
The Captain
Pros:
Modes(Soft, Norm, Hard, and User)
The User mode is a lot like the Vaporesso OMNI board and Tesla Steampunk Nano 120 to where you can curve your wattage by each second. Great when using RTA/RDTA.
The amount of information that is displayed on screen.
The feel is great in the hand with a big square firing button on the side.
Will accommodate a 31mm tank with no overhang.
The option to use dual 20700 or 18650 batteries.
(There is an option with some vendors to only get the 18650 sleeve or 2x20700 and the sleeve.
There are optional side stickers to change if you want or don't like the carbon fibre stock ones. You also can change the back face plate
Cons:
The faceplate and backplate can scuff easily.
The hinge on the battery door could have been a bit tighter(like the Steampunk Nano is kinda loose).
The firmware update can be confusing at first( you need to download the installation.exe file first the the firmware update file. It took me 2 tries)
Both are good devices and when it comes down to it it's up to you. The size, shape, options, etc.
Hth
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