i always prefer a stiff putter.
the stroke of the short arm technique is very quick and usually in a somewhat vertical direction. soft putters will often bend a bit and cause off-axis torquing during this motion. putters that are firm around the rim but soft in the flight plate have a similar problem if you putt with any grip with fingers extended under the flight plate, they flex a bit and make it more difficult to give a clean release.
the last thing i have found, especially when you contrast putters of the same mold in different plastic, are that softer plastics generally have more low speed fade when they drop (a lot of air friction = greater deceleration of spin).
examples of this include the Swirl APX vs. X APX, Omega SS vs. Aviar AP, and JK Aviar vs. KC Aviar or Classic Aviar.
personally, when my putts are on i almost always drill the center of the pole, therefore having a grippy putter really doesn't give me any kind of benefit.