is one for a true beginner, and the other for advanced beginner?
If anything, it's the other way around. Somebody who has a bit of experience will not be able to do 5x5 (assuming straight and heavy sets) squats three times a week. A beginner where weight is generally much lighter and most strength gains are neurological anyway, can handle quite a bit of volume at first though.
This is why I generally start beginners off with a very basic routine where they repeat the same lifts every day. For example, rather than switching between bench and OH press or rows and squats, I'd have the individual do do both on the same day. As a beginner they are able to handle such a frequency so it is best to take advantage of this for best results. Frequently doing lifts will help with form as well. That's why I actually generally don't start people off with Rippetoe. Yes, they squat 3x a week, but they also some weeks only bench once while others twice. Over time I would move them to a more Rippetoe-type routine, however. When I think the frequency is too much for them.
If what you're currently doing isn't challenging enough the last thing you'd want to do is move to a more intermediate type program. An intermediate program would have less frequency/volume/intensity than a beginner program to facilitate recovery. If you can make progress on a beginner routine, you should stick to that routine. An intermediate routine will only give you slower progress, as at that stage workout to workout strength gains just aren't going to happen and they are designed with that in mind. SS is designed so that you increase the weight every workout. If SS isn't challenging enough, you're not lifting heavy. It's really that simple.
I'm done talking now. I probably babbled, but oh well... hopefully something was useful.
