Okay I'll just shoot out a quick answer. If your speaker is on carpet it will do as stated above. It'll couple for the frequencies below the resonant frequency thereby tightening the bass but it'll allow your speaker including its cabinet to ring more fuller above its resonant frequency.

As far as isolation from the ground it'd about equal.

You'll get leaner tighter bass with spikes but more naturally fuller bass with sorbothane. Sorbothane in my tests unless you use a hell of a lot of it is inferior as far as isolation from floor resonance however and the more sorbothane or denser the pad the more it'll loosen the bass giving it what some would call a more natural bloom but others would call too lean.

It's a matter of taste.

Listen to the frequency that offends you. That's the point of this point to allow for more accurate fine tuning.

You can also use a combination of both if you wanted.