Soul is one of my favourite talents, it was very hard to give up. xd Treants in almost all instances is better than soul in arena. Typically the only time I use soul of the forest is when I'm fighting warlock specific teams.
I can list a number of reasons treants is currently superior, but I'll only share a few examples. Classes i.e. sham/hunter/dk/mage/priest use a lot/majority of globals focused on purging, if they get your 15 sec cd soul proc hot that's pretty huge... Treants may allow you more uptime in bear form for when you're being trained/purged by melee, and it helps once again if your partners are being trained/cleaned. Soul doesn't really have an impact versing teams like god comp, the majority of their burst is when you're deeped into a forever cc chain. If you can get off your treants before you're cc'd, it allows you to have "some" sustained healing during that downtime, and once again your soul hot is easily cleaned