IMO a hard cap would be better for a few reasons.

First if you want to bring macro's into the equation, a soft cap is more beneficial to the macroer. Unless the skill decay is severe, and I'm talking minutes, a macroer can incorporate multiple skills into a single macro. They would be able to train up new skills while still maintaining their current skills to reach well beyond the cap. If it is possible to do it, people will find a way to do so.

Secondly a hard cap would mean an entirely balanced playing field amoungst players. You get your 500 points or whatever and thats that.

Then there is the "pissed off factor". With skill decay I personally would be pissed if my running or swimming leveled up and took me over the cap without my knowing it and than I lost a point from one of my GM'd skills because of it. You may not immediately lose a point to decay once over the cap, but I could easily go a day or two in game and not make a tool cause I was doing other things like traveling across the map for a trade deal or something. To get from 99 to 100 in toolcraft was over 2000 craft attempts(I don't have the exact number I stopped tracking at 2k). There are any number of reasons a person may be online and not actively working on their skills. Since things like swimming, running, and combat skills from defending oneself from animals will constantly be leveling without the player actively wanting them to count toward their soft cap, people could very easily lose points from their high leveled and hard earned skills cause of things they can't control. Thats very likely to piss people off.