The stx ones look a lot like the original lx, and the reason I went xt on my rim brake bikes, was if they are set up well, I found you get no sharpness to the initial bite, no squeaking, which may also be pads of course, ( I use ritchey or kool stop cartridge pads.. ) and the levers are the nicest levers, prior to this, as a lad at college back in the day I bought some avid levers, which are very similar and certainly much better than the stx ones, significantly cheaper than xt ones, still are I expect.
So depending on use, I agree in terms of ultimate stopping no difference, maybe a little less refined... but I couldn't use the stx levers as I cannot stand them...! I think the avid levers came with the 1.9 basic avid v brakes of the late nineties..