You really need to look at your usage. If mainly highway/on road then the smaller overall diameter will give you better performance and economy while the higher aspect tyres will not perform or give you the handling/grip that the lower aspect ratio tyres will. If you do a lot of sand work then the 75 series tyres will give you a much longer footprint, which is, of course, is better.....but remember the on road handling trade off.
Also consider that the 18s will be much more expensive and harder to replace if you damage one out in the sticks.
What model do you have? Are your wheels wide enough to take the 285s?
Most people go for 265s which, depending on tyre brand, generally come in slightly over, but not enough that should be a problem - it's not like you're trying to fit 35s that scream out to every copper that they're too big.
If work is so terrific, why do they have to pay us to do it?