Dear Larry,
For the realm of forces in poutting, polymer inserts have as their main effect a prolongation of contact between ball and putter face during the impact zone. This prolongation in time (as much as doubling the period) has a few spin-off effects.
One is "impulse" increase. The amount of work done over time is "impulse" -- more time at work like a factory piece-goods machine operator results in more finished widgets in the bin for shipping, even though the rate of production is the same as other workers This means that inserts don't really kill distance as much as people suppose, just from a physics viewpoint. In your case, I suspect that if you robotized a stroke and put the two putters thru the same stroke with the same impact velocity, you would get very similar distances. So why does the ball not roll as far in your experience? Perhaps you are subconsciously being a little "cautious" with the insert putter when you should just forget the insert and "let 'er fly" as usual. Why "cautious"? because you're a bit of a perfectionist and the insert putter is a new instrument and is not fully incorporated into your playing / artistic "comfort zone" just yet, like a new German violin -- fine, just not an Italian model from the 1700s.
Another effect is on line. The insert "molds" itself at the impact point somewhat to "cradle" the shape of the ball at the impact point, like placing a bowling ball on a mattress instead of on a floor as occurs with a metal face. This "cradling" would probably be illegal if the USGA focues on it, as "concave" faces are illegal due to their directional control features. The old Sun-Weapon of Archimedes at Syracuse was like the focal geometry of a telescope for concentrating the parallel rays at a single focal point in front of the mirror a certain distance out. The "cradling" insert does something similar. The Rule about the stroke (as opposed to the Rule about the equipment design) has a similar prohibition against "spooning" the ball being an illegal "stroke". A "spoon" is a concave shape that also prolongs contact and biases line -- more of a baseball shortstop's making a gloved-hand toss of the ball to the nearby second baseman for a quick double-play starting at second base followed by a leaping throw over the charging runner to beat the runner down the first-base line.
As far as softness of insert is concerned, the insert by the USGA cannot be any softer than the allowable softness of golf balls.
When it comes to drivers, however, the whole world of COR technology comes from Srixon, where Dr Tetsuo Yamaguchi has discovered that more "give" in the driver face produces more "trampoline-effect" and "go" in the ball for distance. This discovery alarmed the USGA, as it opened a new technology that threatened the existing courses. So, inserts are a bit complicated and not really fully understood quite yet.
For Dr Yamaguchi, see:
http://rheo.t.u-tokyo.ac.jp/~doi/publication_theme_new.htm
http://www.pga.com/equipment/equipment-showcase/srixon082404.cfm
http://www.srixon.com/pr_08_09_05d.html
http://www.golftransactions.com/exhibitor/0067.php
Cheers!