The choice usually comes down to how long you need cover and how many sites you run. Hire suits a defined term with no capital outlay and support included. Purchase suits a permanent yard or a contractor who can redeploy a unit across a programme of work.
Questions to ask
How many months of cover do you need in the next two years? Who will move, maintain and insure the unit? Do you need thermal detection or is optical enough? Answering these makes the decision straightforward.