The difference between freebase nicotine and nicotine salt is simply how it affects your vaping experience.
A noticeable difference is a harshness caused by higher nicotine strengths. Freebase nicotine has a higher pH level that affects the alkalinity, which makes it harsher as you climb the nicotine ladder. Whereas, nicotine salt has lower alkalinity due to the benzoic acid which gives you a reportedly smoother throat hit.
Freebase nicotine is recommended for use with high powered vape devices, such as sub-ohm kits.
Nicotine salt is recommended for use with low powered vape devices such as pod kits.