However tempted you are to run Cat5 or Cat6 between buildings, DON'T.
Apart from the issues of lightning strikes and proximity to power cables causing interference, you can get surprisingly high differences in local earth potential due to varying soil/ground conductivity
Whilst equipment will probably handle the voltages involved, the poor schmuck who happens to put his hand on the plug won't. This may not happen for years - if ever. but it's a risk worth avoiding. Yes, there are supposed to be isolation transformers in the transceivers. Don't rely on them being there or being rated appropriately.
In addition, copper cables buried in the ground (or run overhead) will eventually suffer water ingress and corrode. Fiber-optic is immune to this failure mode.
Outdoor fiber-optic cable is cheaper than you might think, as are media converters.
Never direct-bury, for the reasons already elaborated in other posts.
It is worth putting a tracer wire in the duct as this makes it a lot easier to use cable location kit later on (eg, CAT4 and Genny). This can be anything as long as it's conductive and corrosion-resistant (unconnected at either end). You can trace a non-conductive fiber but it's a lot harder as the coupled signal is almost non-existant - personal experience having to trace a 1000m telco fiber run that had been laid 25 years previously with inadequate documentation.
I've seen people kill 2 birds with one stone on this front by using electric fence string as the draw wire. Personally I wouldn't recommend it.
The old "Do it right or do it over" cliche comes into play here. Labour costs outweigh the materials, so skimping out is simply uneconomic, long-term.