If you have a secret, and get caught doing ProHormones, its no big deal. If you have a TS, you'll loose it if you get caught. This allows the military to reclass you against your will. In addition, when you go for a TS, you'll have to admit to PH/Steroid use, as well as when. When they match the timeline up, and realize you violated DoD policies, the chance of them granting you one is fairly slim, though has been known to happen. If you did it as a civilian, its not a problem.
In case anyone is wondering, i was in the S-2 shop, and we pulled clearances if something shady was happening. Pulling a secret clearance requires you to be absolutely stupid though. You'd have to steal from another soldier in excess of 500 bucks, etc. A TS is a different story though. Some jobs require a secret (communication for example), but when you go for your E-8, you need a TS for the same job. Just throwing it out there, that depending on MOS, you should know the requirements of advancement before you try something.
A secret clearance is them making sure you have no felonies on your record. A TS is a 50-250k investigation about you, your morals, and fact checking your past. There is a HUGE difference.