Depends what you want the board for......
I havent ridden a SP but to me they seem to be a small wave board for milking the best out of small waves.
The Zot on the other hand is probably the best board that I have ever ridden, but as with all McCoys it likes some power in the waves. Think smooth, flowing style with insane ability to turn with such speed.
It will go in small waves but not really in the mush. I have had it in decent size surf and it is so stable and inspires confidence as do the Nuggets.
To answer your question though, I would say that a 5'10 Zot is roughly 40 litres whilst the same size SP is 47 according to the Firewire site so the SP maybe slightly more floaty. Don't be fooled by similair widths and lengths in the wto boards because as Cuttie states the Zot doesnt have such a full nose as the SP and I would actually say that there is more similarity between a small Potbelly and the SP as the Zot is more Nugget like upfront.
I've had two Zots (still own one) and currently tempted by a smaller version, so if you get a sub 6'er and dont like it I'll take it off your hands