You CAN get over 100 Mbps at 300m on coax using Moca adapters. This may get pricey as the technology is pretty new.
http://www.mocalliance.org/en/index.aspThe issue with all networking technologies is that you will almost certainly be violating the ANSI/CSA/UL 60950-1 listing when you run between buildings. The interface ports of most networking equipment are classified SELV and as such they are not subject to the overvoltage testing (power line cross) that is required in the US and Canada. In other coutries you will only be violating circuit separation clauses of the safety standard. When you run these indoor circuits outside, this make the interface a "cable distribution system" (rather than jsut a SELV circuit that happens to connect to coax) which adds testing requirements that may not have been considered during the NRTL investigation.
If I was uncertain of the conditions of the product Listing (and unable to stop myself from proceeding with the installation anyway), I would install the equipment on and over a non-flamable surface and/or locate it inside a fire enclosure. I only say this cuz I know some people are just going to run stuff outside anyway. May as well make some effort to encourage them to do it the least wrong way possible.
Ethernet will have the same violation of Listing issue.
