I love both and both are definite must-sees. However, for me the Osbourne Lights is a game-changer/once in a lifetime kind of extraordinary performance and just unlike anything you cans ee anywhere. The Candlelight is a truly wonderful rendition of the Nativity and excellent...however, there are tons and tons of variations on Christmas Pageants like that (well, of course, not exactly like that, but ballpark) that you can see anywhere. Disney does a great choral and narrative and colorful rendition of the Nativity...but they do not reinvent the wheel here.
For the Osbourne Lights, the wheel is not just reinvented, it is smashed and made new again, unlike anything you can imagine. It's exciting, it's dazzling, it will leave you in awe. It's a combination of the best Christmas music set to dancing light displays...and that description can not do it justice. It's a party, it's festive, it's Disney. The Candlelight is more sedate, more like Church, albeit quite enthralling and beautiful. It's not exactly...fun though.
By all means, if you have time this season, do both! However, even if Osbourne wasn't going away, and you could only do one, it still wins because you just can't get an experience like that anywhere else and it's atmosphere is 2nd to none.