Before you move forward, let's get something straight. There are 3 models of 525 trimmer:
525L: This is the base model. (Base isn't the best term to use, all of these trimmers are high end commercial units.)
525LS: This is the heavy duty model. It has a solid driveshaft, metal engine guard, and it is 0.13 HP more than the 525L. Don't know why, as the engine parts are exactly the same. :confused2:
525LST: This is basically a 525LS with a different gearbox. The gearbox on this model is geared lower, which means that the trimmer head spins slower but has more torque.
You don't need the LST unless you're going to cut brush or remove the operator guard to run longer line. (I don't recommend removing the operator guard, but some people do it anyway.) If you try to run the LST like a regular trimmer, you'll be annoyed with the results. You will end up using a lot more gas and the strings won't cut as clean due to decreased speed.
If you're just doing regular trimming, the 525L should be just fine. For the record, it's the one I bought and I have more than enough power for 99% of the jobs I do.