I find the Words program best for composition, especially if you are trying to compose about "modern" things, e.g. computers, hamburgers, shovels. However, as far as desk dictionaries go, outside of the pain of doing Bradley's Arnold in Grad school, I have never done too much with composition, so I am not that familiar with strengths in this area. So, I would still go with the recs above. I tell you, 9 times out of 10 , I am using New College.
For Composition, I would find a book with a good dictionary in the back (North and Hillard's Latin Comp, for example.)
PS. Cassells has an English to Latin section, though I don't remember it being very impressive. I would simply look at my copy but I think it is at my office.
When a true genius appears, you can know him by this sign: that all the dunces are in a confederacy against him. ~Swift