The front is the better choice. There are a few reasons for this:

The front brake is responsible for most of your braking, and it is capable of stopping the bike much faster that the rear brake. Also, if your frame and fork are not equipped with disk mounts, you either need to modify or replace the frame or fork... well a replacement disk compatible fork can be ~$50, but usually a replacement frame is more expensive and a much more difficult job. Also, if you don't have disk compatible wheels, a front wheel with a disk mount is cheaper than a rear wheel. But all the other reasons do not make up for the fact that your front brake is more important, should be used more, and is more effective at stopping the bike.

Although, a properly set up V-brake with good pads and new cables is perfectly adequate for almost all riding.