The SPN rulebook is available online at:
http://www.slo-pitch.com/PPP/card6/c620.aspx?r=0
I don't know ASA other than what I observe by lurking here, but some quirks about SPN include:
Commitment Line:
-is not used in all categories of play, but when it is:
-the runner is committed home once he/she touches the line or any ground beyond the line (in Softball Canada, the runner must touch down with one foot entirely beyond the line to be committed).
-once committed, the runner may not return to 3B for any reason (in Softball Canada, the runner may return to tag up on a caught fly ball or for a missed base).
Scoring Line:
-is used whenever the commitment line is used, in which case touching home plate is an automatic out regardless of whether there was a play. Sliding at the scoring line is also not allowed.
Pitching Zone:
-is a zone 2' wide x 15' long starting at 50' from home (and extending out to 65'); pitcher may pitch with one foot firmly planted anywhere within this zone (there is no pitching rubber).
-note that the pitching arc is 6' to 12' (same as for Softball Canada and, I believe, ASA), but many leagues around here override this with either 6'-15' or 6'-unlimited.
Double 1B:
-no allowance for fielder and batter-runner to switch colours even if the play is from foul territory (i.e. F must always use the fair portion; BR must use foul portion on first attempt if played on).
I'm sure there are other little differences, but this is all I have from memory.