Those are both great tools, so it's really a matter of what you prefer working on.
I own a studio, and in that studio we use Nuendo...for tracking and mixing, it's such a powerful thing...and Cubase is a lot like Nuendo, so there's one argument.
I am also a composer/producer and for my laptop, which is used primarily for writing/preproduction I use Logic because it's fun to use after you learn it and nothing comes stock with so many great plugs and instruments...I think Logic 8 is under $600.00 and you get a guitar amp sim, bass amp sim, lots of great synths, electric pianos, a sampler, a great drum machine, a great Organ sim, an awesome Delay plug, an awesome reverb plug, really good eq's and comps and a truckload of apple loops, which can be dropped into any project and will automatically adjust to time and pitch...to buy a Cubase rig with so many great plugs and instruments would cost at least $2,000 I'm guessing...With Logic, everything is right there and you're off to the races...so bang for the buck, nothing touches it, IMO.
My engineer doesn't need tons of instruments or MIDI in general, so he loves Nuendo...but again it depends on your needs and budget. With good converters, soundcard and plugs, you're talking about a lot of money...Probably $6k? Probably more actually...for my writing rig, not counting the computer, I have Logic 8 and an Apogee Duet...ok, only 2 ins and outs, but, great sound and results for like $1,200 tops.
Did that even help?
-Mark