Most airfields in the UK have little security.
We had a Helicopter owner who spotted a small dark metallic smere aroung the oil tank filler (500D) when going to check his oil level first flight in the morning.
Some kind soul had topped up his oil tank with grinding paste. The smere was hardly visible and would not have been seen by someone not checking the oil.
Then there was the 150 pilot who noticed an unusual tincanning effect on the left elevator.
During the night someone had forced the elevator against the stops damaging the elevator mountings. Again hardly visible at first glance.
Blocked pitot tubes and urine in fuel tanks are other pre flight finds I have had to deal with.
Probably the worst was a house brick wedged in the nose gear of a Seneca, not visible without bending down for a good look.
If you have no enemies, all the locals love aviation and there are no pubs within walking distance well and good.
Personally I would go for the preflight every time.