Particularly these days you have to be pretty far off the beaten path to find yourself somewhere that charging off the 3-phase socket at the showground is your bets option.That means I also recommend installing a 5 pin socket at home, even if you only have single phase - so you can use the adapter in one of the ubiquitous 5 pin sockets out in the wild, but if you really want to install a 3 pin then the choice is between
a) the round-pin which at least holds the plug in securely but is useless for everything else, or
b) the flat pin which you can at least use with 10A and 15A plugs but which risks the plug partially coming out and causing arcing if you're not careful.
I liked the round-pin socket because I liked the way it seals when not in use, protecting the components from corrosion, as you say the screw collar is nicely secure, and I feel the electrical connection to the round pins is more durable than the flat bladed ones.
(There's a few dedicated threads for charging adaptors, including the sticky one Charging Adaptors 101).