I don't know much myself but I spoke to a friend who has Speedtree at work. He said two things basically.
1) Speedtree over Xfrog in a heartbeat.
2) IF you use C4D/Maya then the plugin over the Win standalone, although he almost had a stroke when he found out the plugin is about $50-60 more expensive.
He also said that Speedtree is WAY more comprehensive, although with that comes the added expense, but he'd never advise a NEW USER to start with Xfrog. If you've got experience with it and then you can get decent results out of it but if you're a newcomer than it's hard to justify choosing what's essentially an inferior product.
My 2c is a little odd. I don't like Xfrog's website. It looks dated. I know this isn't the end all be all to development but...
...if they can't come out of the early 2000's there, then have they in their software?