The fundamental differences between the two are that cubase is a high end sequencer and is predominately intended for creating music at the mixing stage, so is good for multi-track recording and hosting virtual synthesisers, drum machines and audio effects, whereas wavelab is aimed at audio processing at the mastering stage, which is normally done after all the mixing (say in cubase) has already been done. Wavelab can also host virtual effects eg vst or directX plugins, but is typically used to process just ONE section/track of audio.
The fact is that there are crossovers in most of the two programs capabilties, for example cubase is perfectly capable of mastering as well as mixing, simply by applying mastering effects to the main output bus, similarly i believe (although have never actually tried) using muliple tracks of audio in wavelab is also possible.
It really depends on what you want to do, I would say most requirements are satisfied by cubase, like recording a band, making any genre of electronic music, using your computer in a live music gig. Wavelab is as i say, good for mastering an already mixed track, or putting together a cd album of completed tracks.