BigSpencer,
I'd research this one carefully... unfortunately you will probably not get the chance to cast the rods side by side before you buy them.
Here are my conclusions from my experiences: For Sage SP 9' 5wt - the 3pc is a step slower than the 2pc... I am going to assume this holds true for the other line weights. Sage SP 4wt, 5wt, and 6wt are a faster action than the Sage SP 3wt. (From handling 4,5,6wt in 9' models and handling 3wt in 8'9" and 7'6" models).
Sage SPL was noticably slower (more flex) than Sage SP (When comparing 8'2" 2wt and 8'9" 3wt SPL's to 9' 4wt, 5wt, and 6wt SP's).
However, I believe the 8'9" 3wt SPL and 8'9" SP may be closer to the same action... unfortunately I handled the two rods about six months apart from each other.
So generally speaking - Sage SPL is softer than SP. Sage SP in 3wt is softer than Sage SP in higher weights. Sage SPL 3wt and Sage SP 3wt may be closer in action to one another when comparing the SPL to SP.
My .02,
CG