I put 'none of the above', because 1) I play(ed) both RM2 and RMSS, and 2) it was okay either way. As others have said, in RMSS you can cast Bladeturn and do other stuff, and so -100 would really amp up someone who chooses to also attack while casting it. OTOH in RM2/RMC a mere -50 for something that blows a bunch of PP and is effectively your whole turn is quite weak (and if spending your whole turn and those PP doesn't do a lot more than a 100% parry, what's the point?), whereas at -100 it's worth doing - most attackers will miss.
Edit - as to why Deflections stayed at -100, consider that (assuming my memory is working right) it is your one spell for the round, and most spell casters won't have much (if any) skill with a missile weapon, so if the use Deflections for that -100, they can't counter attack, even though in RMSS the casting was only a 10% activity. Thus in RMSS it needs to remain at -100 because it still effectively prevents an attack.