The difference between Street-level and Street Scum (outside of story reasons) are these.
- Street Level assumes that you are well skilled/trained etc, but haven't had enough jobs to acquire Karma and Money. Most people don't afford 300,000+ nuyen in 'ware without some runs under their belt.
- Street Scum assumes that, because of your situation and environment, you are not as skilled/trained, but you still have experience in the "real" world. Hence the karma and Priority arrangement.
The practical difference between them is that Street Level itself is horridly unbalanced and favors Adepts, Magicians, and Technomancers (but not Mystic Adepts) over everything else. Street Scum, on the other hand, is pretty well balanced, although some archetypes still have trouble fitting everything in. Street Scum does a much better job at making Shadowrun feel like Street Level was supposed to; characters who are just starting out and still honing their abilities.
So, yeah, most people like Street Scum a lot. Also, yes, Street Scum and High Life (which I really like, except they need to bump up nuyen levels to compensate people who depend on 'ware as opposed to magic) needed to be in the Chargen section, and they are really much better balanced.