It depends on where you're coming from, your finances & what you prefer really.
Train is fine but Temple Meads is chaos at the best of time & it's even more insane when Glastonbury is on. You also have to get a bus connection from Castle Carey to the site & that's a pain.
Coach is fine but if you're travelling a distance be prepared for getting stuck in roadworks. Car is ideal as you can bring pretty much everything you need with you but obviously you have environmental concerns, but car sharing is obviously preferable.
So each method has it's pros & cons.