People seem to get some pleasure out of writing them though
I'd say that's the guts of it. Do whatever makes you feel good.
I update (rather than keep) a cycling diary periodically. Just a document of my rides; how I felt, what I say etc. It's not stats or goal based. I've no motivation to put it online though. I've read a number of blogs that seem to be personal diaries and they give me no desire to follow them. I do like a well written blog that with an awareness of an audience though. They can be informative even if only personal opinion based. From my browsing, I'd say most blogs don't fall into that category though.
I like forums (for the most part) as I like the debate aspect. And you can learn from knowledgeable people that you otherwise would not come across, ask them questions etc. I like participating so long as it doesn't start getting aggressive. I'd actually say that I've changed my approach to forums too, and I'd say that has broadly reshaped my temperament in life too... but that's rather a large claim to make for forum participation! I'm sure most people wouldn't go that far.
But, yes, different beasts. Enjoy both for their differences. I'd say the biggest difference is the level of engagement. Responses to a blog are fewer and less immediate. Forums are more vibrant, if I can put it like that.
