While talking about playing in our respective school orchestras, a grocery store clerk asked me recently, "If you had to choose between being able to sing well or play an instrument well, which would you choose?"
I hesitated only slightly before answering, "I'd want to play an instrument well." That's how life panned out for me anyway. I had 10 years of reasonably satisfying, successful bass playing. Still, just for a moment, I contemplated what it might have been like to sing well. I've never thought about it seriously, as I've always had a limited singing range. Being able to sing your favorite songs, especially ones you've composed, while playing the guitar or keyboard must be especially gratifying for those who welcome the limelight.
Overall, though, I think I'll stick with my answer of preferring to play an instrument well. Generally, I think one's ability to play an instrument lasts longer than one's ability to sing.