Well Poker Tracker is different than SS. SS also cannot be used on PS for example. SS uses a central database to gather info on people, then pulls that info up in front of you. PT allows you to do something of the same thing, as long as you gathered all of the info. The benefit is that PT also allows you to find leaks in your game and fix them. You can replay hands, see all kinds of stats on different hands/tourneys etc. I think you are better with PT (which you can download free and try the beta version right now). If you really want ROI stats on people, you dont need to pay for SS, just go onto OPR (official poker rankings). It is a bit more work that way, but you get the same info as SS offers for the most part.
Edit: I am not sure if it is current, but I see that it says you play at PS. You will get into trouble using SS there, as it is now banned.