If it were me, I'd choose a "niche" blog. I place a greater value on detailed content of a particular "niche" topic rather than reading though a generic blog of ramblings.

Market analysis says that if you are doing a general blog you have to compete with several thousand other bloggers to provide specific value. This is a daunting task. If you go niche, it depends on the niche, but you would probably be looking at more like a dozen direct competitors for the eyes of the readers. It often makes sense to start out niche.

This isn't necessarily a decision you have to make right now. I'd just start writing interesting posts. Use analytics to track which posts are generating traffic and use that information to make a more informed decision about the direction your blog should take.

Having said all that, I agree with the other answers saying that a niche probably will end up being the way to go, but there's no reason not to play around with different ideas.