Well, I wrote a bit about what musicologists do here. In terms of research areas, I myself am the score-analyzing type of musicologist, so I spend my days analyzing music and writing about my findings. I'm an academic, so teaching is ordinarily a large part of what I do, although this year I have a fellowship that lets me do research full-time. Pseudonymity prevents me from saying more in public about what I research, although I could go into it by PM if you are really interested.

I am (well, was -- I don't play much any more) what I once described as a "low professional-level [classical] pianist." That is, I play classical piano really well by most standards, but would never have gotten famous. At a much lower level, I can also play jazz piano and Baroque harpsichord. I never learned to play organ, and never learned any non-keyboard instruments. Among professional musicologists, I'm pretty much average for both number of instruments I can play and level of skill.

As to pieces about Jupiter, I can only offer you my personal opinion -- being a musicologist doesn't make my musical preferences more valid than yours. Both pieces are great, and I had a special fondness for the Holst when I was a kid (I heard it in a concert hall when I was about 11, and spent the whole 40 minutes grinning hard enough I should have burst a blood vessel). But I'll take the Jupiter Symphony without the slightest hesitation. Here you have one of the greatest works of one of the tiny handful of greatest composers ever, versus an excellent piece by a one-hit wonder among classical composers.

Really, though, I don't much like picking favorites among pieces of music, and always want to preface my answers with "Thank goodness I don't really have to choose!"