I work for WANdisco, and I will explain why svnsync will not meet your needs for implementing HA Subversion.
The downside of svnsync is that if your subversion server goes down, then you will have no write access to the repository, and the svnsync copy of the data will be out of date. The only way to guarantee HA is to have a fully working, up-to-date copy of the repository on a second server which can be put into play immediately when the main Subversion server goes down.
In this rispect WANdisco replication technology is unique - the active-active replication technology allows for real-time replication of any changes to the backup svn server: not a scheduled replication like you would have with other solutions.
Regards,
Jon