The longer, softer, spring provides a more constant level of resistance throughout the working range of the mechanism.
Spring selection for a Steyr will be quite a bit different than selecting one for most conventional autoloaders. In conventional designs the hammer helps hold the slide in battery during firing and the work of cocking the firing mechanism is fully accomplished during the back stroke of the slide. In the Steyr the recoil spring has to provide all of the force to hold the slide and barrel in battery AND the recoil spring also has to do the work of pre-loading the firing mechanism. Both of these factors place a premium on having a recoil spring which generates a fairly high level of force when the slide is in battery.
Bottom line: A shorter, stiffer, spring is more likely to produce out-of-battery problems and/or excessive force near the rear of slide travel (short stroking). There may be profit in tinkering with the spring rate a bit but I would be skeptical about shortening the spring by a great amount.
I have no experience with Glocks, but, because of the similarities in function, that's the first direction I'd look for alternative recoil springs. The possible dimensional differences may dictate the use of different guide rods with them or may rule them out altogether, though.