My honest opinion from use of all 3....
Cubase...great GUI, Easy to pick up on composing music in it, not the best sound library(instruments or FX), but adding vstis fixes that, Great Multitracking.
Logic...Easiest to compose on, very short learning curve, all the effects/sounds you'll ever need(even though sounds are overprocessed), just seems to lack that "professionalism"(for lack of better wording) I look for in a DAW. Reminds me of Cool Edit pro. Weird because older vers. of Logic were nothing like that.
SONAR...Best of the 3 at multitrack recording, best instruments by far(but logic has better FX that cover everything you'll ever need), but the biggest headache of all 3 when it comes to composing.
Again, just my opinon.