Here's a 10-minute video outlining the library topic for 9/30, which compared and contrasted Oxford Music Online (also called Grove Music Online) and Wikipedia. Be forewarned, I recorded this in my office, and you can hear the hum of fluorescent lights in the background.

In summary, OMO is reliable, written by experts, and often contains very useful technical details and works lists. However, it does not always include the most up-to-date research. It is harder to gauge the reliability and expertise of Wikipedia authors, but since articles can be readily updated, Wikipedia will often reference more recent research than OMO.