At 300 ft I would definetly aim to cable it. Is this for a business that can afford higher end stuff or for personal use where an ROI will not likely happen?
Assuming personal use.
Is there a road or anything in between the building that would stop cables from being buried? Are there any poles to use to put cabling up high?
I would avoid Cat5/6 cabling. The spec is 100 meters between. That is the spec but in a pinch I've seen the cabling work 10-15 meters longer. However, you're saying 300 foot between buildings, and that won't account for cable routing or if the router/switch is on the otherside of the building.
Given the information I have, I'd probably recommend the following solutions.
1. get 3 switches that have fiber (I think multimode will work, but I'm not a fiber guru) interfaces and place those in the building. Run fiber (with casing/conduit) between the buildings.
2. 3 switches, cat 5/6 in between them and hope for the best.
3. You can try wireless.
none of the standards will reach 300 foot, even without buildings/trees and such blocking the path. There are amplifiers that might help, but you also amplify the bad with that. Some sort of microwave tech is what you might need to look into.
Without doing a site survey it is difficult to come up with a good solution. I will say, fiber, underground, in conduit will be prefered. The copper might work but would not be ideal - I doubt it will work well though without a repeater.
You can also get a copper switch for each building a find a fiber/copper transceiver. That may be a little more cost effective for you. If you can provide a little more detail, we might be able to come up with something.
Edit: I misread your post. Change 3 switches...etc to 2 for only 2 buildings.