The big case diff is oem. Diffs came in either a small or larger diff case depending on the model.
Most TT6 prefacelift jspec supra's came with the larger casing diff. You can check if yours has it on the engine bay vin plac, your differential code should start with B if you have a large case, or A if you have a small case, for example B02B or A01B etc.
The issue with all the supra factory LSD's are that they are torsen diffs, which means they generally last a long time and require less maintenance but due to their operation, they are far less aggressive/efficient then proper clutch type LSD's.
Generally speaking a large case oem diff will be stronger than the oem small case diff and can handle more abuse. If you already have a big case diff then you could upgrade to a TRD clutch type (if you can find them) or Kaaz, OS Giken etc etc.
The type of LSD you choose should also reflect the type of driving you expect to put the car through. If you only plan on driving the car on the road, I wouldn't really bother with a diff upgrade if you already have a big case, but if you expect to take it to the track then it could be worthwhile.