Hard-wired is the way to go. It not just is a lot more theft resistant than a box hanging from a pedestal, but it is always on... no forgetting to grab the external one and plug it in.
Even with an EMS, I'd go with a surge suppressor just so that something relatively cheap takes the hit of a nasty spike.
As for securing it, one better method I have seen was just plugging the cord in at the pedestal, threading the cord under the rig, and placing the portable EMS there, on top of a board or block to keep it off the ground (chained to a wheel, of course.) Not 100% secure, but the key is both having a theft deterrent, as well as having it not visible to a would be "borrower" in the first place.