Jeepers! Calm down, probably should have qualified my comment. Why does it matter? One is about offsetting your guilt (like corporations planting a tree for every tonne of co2 they produce) and the other is about a lifestyle choice. Yes the two can go hand in hand but often they don't. Its important to be honest about why you do stuff. The end result might be the same but there's an element of personal development isn't there? Sometimes giving money is every bit as important, kind if depends on the needs of the project.
At the moment I do **** all for anyone other than myself, my wife and my kids. In the past I've worked as a volunteer in a homeless charity, cleared ponds, river banks and beaches for a nature conservancy group and done a fair bit of marshalling at local events etc. At the moment whatever limited free time I have is mine. I've earned it by getting up at 5 every day and working a 12 hour day and studying for an MA at the same time. I have no guilt. No doubt when things have calmed down a bit in my life and both my lads are at primary school I'll get involved there.
Edit: I work in further education, and yes it was a deliberate choice to work in a sector that benefits society.