My apologies for thinking it was Nuendo, I thought maybe you got in over your head and were looking for something a little easier.
I can say that for the majority of home recorders looking to do nothing more than record their own tunes in a semi-professional system that Cubasis probably does just fine. One thing Cubasis seems to lack in is real heavy support from the company (at least from what people write here) because they are most concerned with their Nuendo and Cubase users, which the software is much more pricey. One thing to investigate is how many simultanious tracks you can record at once with Cubasis, there may be a limit, I am not sure but I thought I remembered there was. It could make it hard to record live drums.
I got in with Cubase VST32 and upgraded to SX and I have found that between reading posts here, and at cubase.net, and the manual that I have not had any real trouble with SX. Everytime I did I was able to get myself out of it by reading the manual. Contrary to many people's posts, I find the manual really is clear and concise and to the point. The hardest thing by far was getting my system optimized in XP and that had nothing to do with SX. I've now have had it for two months with NO PROBLEMS! I am psyched. All the tweeks are done.
SL is the "light" version of SX from what I understand. I don't know the specifics of what it's limitations are, but it is much more advanced than Cubasis.
I believe upgrades are available from either program. Check Stienbergs sights for upgrad prices.
Good luck and let us know if you have any more questions.