Attack of the Ti + Ni!
It's been my observation that traits are found in loops, rings, however you want to visualize it. There are two opposing sides to everything, and these two sides are complementary. This can be found both in individuals and in interpersonal interactions. I concur with the usage of the NP, NJ, SP, and SJ system, and find that the NPs and SJs go together and the NJs and SPs go together. Specifically, NFPs and STJS, NTPs and SFJs, NFJs and STPS, NTJs and SFPS. Those familiar with Socionics will recognize this as a breakdown of the quadras.
In all of the descriptions listed above, the groups are described as separate entities.
I see SJs and NPs as a conservatory force- with the SJs maintaining that which is (Si), whether it's logistically or practically focused (Te - STJs) or people focused (Fe - SFJs) and the NPs analyzing the possibilities and perfecting that which is (Ne), whether to a logical standard (Ti - NTPs) or to an ethical standard (Fi - NFPs).
Likewise, I see SPs and NJs as a transitory force- with the NJs, you have their Ni orienting them towards visions- they see the future, predict it, and usually have an idea for how they want it to go, whether it's logistically focused (Te - NTJs) or people focused (Fe - NFJs) (Notice the parallel between SJs and NJs), and SPs are enactors, STPs skilled with the logical implementation and SFPs skilled with ethical implementation.
So, to answer your more specific question, "What is the difference between NP and SP", as a general answer, I will present it to you this way:
Both SPs and NPs have a strong internal sense of either logic (xxTP) or ethics (xxFP), the main difference lies in that SPs are more concerned with enacting those things in the here and now (Se utilization), while the NPs are more oriented towards idealizing situations or coming up with ways it *could* be enacted (Ne utilization).
To get a further picture, I'd suggest lining them up side by side, like so:
ENFP vs. ESFP
INFP vs. ISFP
ENTP vs. ESTP
INTP vs. ISTP
Since I am an ISTP, I will use this as an example. If you compare my type to the INTP (Ti doms with differing auxiliary), you find that the ISTP's Se orients their logic to a more concrete, practical form. We are not content to just analyze and organize things- we want to see it done. I'm not content until I at *least* explain the system to someone else. INTP's Ne orients their logic to more theoretical realms, alternatively generating and analyzing ideas to find which is best. I apologize for the shorter explanation for the INTP, as I have a hard time understanding and explaining Ne. This should be enough to get the idea, though,