I have all three. Collie X300B (GPS version with 4 x paddle antenae), Spitx X750v2 and Puli XE300
Price - all three
Portability - probably the Puli, as no antennae to connect and bulit in battery) but not a lot in it.
Power bank - probably, 1.5A should be enough.
External antennae making a difference - I find the Collie better than both and the Spitz better than the Puli The Collie is excellent for 4G reception and also has a very strong WiFi range compared to both (off topic > just shame it doesn't have a USB port!).
Plug 'n' Play - Yes, all the same (except connecting the antennae)
Setting-up VPN - all the same
LTE fallover - no idea.
Hope this helps.
PS. don't know how the Collie GPS with paddle antennae compares to the other models with plastic.