They're both really good but I prefer Logic.
IMHO, it's more focused to begin with - Cubase is really modular in terms of windows - Click this to access this button to open that window. Logic is more similar to Sonar and Studio One. (Well, actually, they both "borrowed" their GUI layout from Logic.)
The scalable UI also helped sell me on it - we were hitting the road for 2 years and there's no way Sonar would have worked on a little laptop screen. Though I was glad to finally hook up to a larger monitor when we finally arrived home, screen real-estate was not so much of a concern during those 2 years.
Logic has an integrated sampler, EXS-24, and the accompanying tools, including the audio editor. Simply put, if you're into sampling, nothing comes close to Logic. And since it's integrated, transforming an audio clip into a sampler instrument is a matter of one click. EXS-24 is crucial to my workflow. I can do anything with it, from sketching songs to full-fldged finished pieces of music using nothing but EXS-24 and Logic's native plug-ins.
Speaking of which...
The quantity and quality of plug-ins. To this day, Logic's compressor is still my favorite, no matter how many 3rd party I own. And there are plenty of interesting, less orthodox and creative effects. The organ and electric piano are as good as anything on the market. Ultrabeat is a crazy drum plug-in, though the GUI can be intimidating at first. They're all very good or great, really.
I also need to add the factory content - almost 40GB of it, and imho, far beyond anything provided w/ any other DAW. That stuff is actually usable in a finished project. I've created quite a few pieces using nothing but the bundled samples. Some of those instruments are simply irreplaceable to me.
Track comping - the best there is out there. Apple
invented swipe comping.
One thing I use quite a bit is the ability to import session data into the current project - could be audio tracks and their setting and plug-ins or tempo maps or markers, or whatever. To me that feature is crucial and I don't think there's anything similar in Cubase and the others. I did a little video about it a few months back to show the guys here what I meant. Here it is.
http://www.youtube.com/watch?v=Cr8fN2MCMvU This really only scratches the surface, really. There are so many little things, so many details. For example, if you look inside a project folder, you'll see an actual image of the project. This may seem silly, but I can't tell you how often in the past I've had to load a project just to figure out which of the 15 different versions was which - and as soon as it was on screen, I knew I had the wrong one. That's the kind of little detail, the level of attention...
I like Cubase and I've used and liked Cakewalk Pro Audio and Sonar for so long. But to me, Logic is exactly what a DAW should be. Not on paper but very concretely. It's like it's always thinking one step ahead of me, sorting out the options.
One of the best example is the aforementioned swipe comping. While the other guys were implementing track lanes and solo/mute/merge tools and creating all those options to audition and splice and glue and recombine your takes, Apple asked themselves what it was that we really wanted to do - and the answer is:
select the best part of each take for playback. That's all. You don't
want to splice or merge, these are just means to get to your real objective. So that's how you comp in Logic - you swipe across the part that you want in each take, period. All the rest is taken care of automatically - even if you can obviously make manual adjustments.
That's one of the reasons why I say that it's always one step ahead.