First off, IMO, there really is no "vs" in this scenario... they are very different apps in both their approach and workflow for music-making, and they manage things very differently. End results often reflect that as well... It would be my advice to first look closely at what your needs are, what you want to accomplish and how... then match the app to those needs?
I use both Live and SX. I quite like both, and my workflow really reflects that... I've gotten tremendous benefit from developing a working process using both apps.
Live has become my de-facto "sketchbook"... whenever new compositions are birthing, that's what I reach for first. As well, when jamming with other (digital) musicians, it's the bomb as a collaborative platform. You simply can't improvise in an app like SX the way you can with Live... that's one of the key things about it that makes it so great.....
Usually though, when I'm ready to progress further, meaning, seriously build on those initial ideas, get into deeper mixing, synch to video, etc. (post-production, basically), that's when Cubase comes into play. I either move the audio tracks over to Cubase via export/import, or rewire into SX..... I have to say that generally the sound quality of the mixes in SX (and my end results) are clearly superior.
That said, I couldn't tell you as clearly WHY that is...

Whether it's just a function of the user interface (a certain kind of 'usability nuance' can indeed impact on one's workflow and end results), or simply how the apps handle audio, I couldn't really say.... it really may just come down to how the apps "work" for me as an individual user...
This is not to say it can't all be done in Live exclusively. I have pieces that never left Live, start to finish, and I'm completely satisfied with them..... so it can definitely be used for complete projects, end to end.
If I had to choose between them as a start point, then I'd probably recommend starting with Live, and 'graduating' to a full-featured DAW later as the need arises (e.g. a more "formal" mixing environment, post-production tools, etc.). There are things you can do very efficiently in apps like SX, Logic, or DP, that just don't fit in the workflow as nicely using Live's approach....
Anyway, I think Live is a great tool, and can fit the bill nicely as a "first DAW".... and this will only be strengthened with v5.....
Again though, identify your needs first, then find the best fitting app... do that and you won't go wrong....
just my two cents, hope it helps!
peace,
tribalogical