They're completely different, really.
Merapi erupts very often in small eruptions. It's never had an eruption that affected much beyond a few miles of itself or that has affected world climate or anything.
Pinatubo completely blows up every 500-1000 years or so.
The largest known eruption of Merapi in the last 10,000 years was only about 1/1000th the size of the Pinatubo eruption in 1991.
The danger of Merapi is the incredibly dense population on and around it; Java is one of the most densely populated areas on earth. Thus you have thousands of people exposed to small pyroclastic flows and such that on most volcanoes wouldn't even get mentioned in the news.
I picked up a couple volcano videos for the kids, at Marshall's of all places, and it turned out one of them was a documentary about the USGS and how it followed the whole eruption of the volcano from when there were first reports of it becoming active until it blew. There was some really impressive footage of the days just before and the actual eruption itself, mostly taken by soldiers at the military base there. The immensity of the whole thing is mind boggling. Fascinating video.