I have a Customer with two buildings across the street from each other in a similiar situation. When I started working with them in 2003 we put in a fixed wireless (802.11b using Cisco Aironet gear at the time) system. It worked well enough, but we had outages when an AP failed, and a loss of performance when the neighbors all started putting up wireless Ethernet and other crud into the 2.4Ghz ISM band (yes-- we were using highly directional Yagi antennas pointed directly at each other, too). The Customer "upgraded" the system to an set of Proxim 802.11g APs in 2005 and saw very little improvement in performance.
When the Customer started running VoIP over the link it became very clear that the link wasn't very reliable and had reasonably unpredictable latency. This traffic, combined with the increasing size of the corpus of files that we were backing-up over the "air" each night from a file server located in the "across the street" location pushed us to recommend fiber.
We had the local cable television monopoly (Time Warner) run the fiber. For the Customer, this meant that they didn't do anything at all with permits, digging, hiring contractors, etc. The cable monopoly got the fiber run, put Ethernet switches at both ends, and told us "plug your network in to this port" on each end. That gave us a 100Mb/sec, rock-solid reliable connection between the buildings.
For the Customer, there is a recurring month-to-month expense. Initially I was opposed to this, however the "pay off" for the Customer paying for the fiber installation themselves versus the recurring charge turned out to be roughly 3 years. The Customer's financial controller also liked the idea of a month-to-month expense, versus a capital expenditure. (You'd have to ask him why... this is Server Fault, not Obscure United States Federal Tax Code Fault...)
Every fixed wireless link I've had the occasion to work with (all of which were installed by other contractors except for the one I've described here) have been problematic in some way or another, as compared to fiber. Fiber, once it's in the ground / air and working, works forever (unless it gets BIFF'd or shot). You'll change the electronics on the ends from time-to-time, but that usually just means an increase in speed or features. The fiber itself remains the same.
I'd recommend strongly against running the fiber yourself. You can save a lot of time by getting together with a cabling contractor who has done this kind of work before. They'll know what permits to get, and have heavy equipment available (like directional boring machines). They can tell you, too, if your buildings are close enough together to get away with using the less expensive multi-mode fiber optic cable versus single-mode (which can cover a lot longer of a distance but is much more expensive, both for the fiber and the electronics on the end).
Check with your local monopoly cable television provider, too. They may be able to run it for you and, depending on how long your company wants to stay in those buildings, what the month-to-month cost is compared to the cost of installing the fiber with your own contractor, and how your finance people feel about an expense versus a capitalized asset, you might found out that the monopoly provider ends up being the route to go.
