There are a few differences between them and even differences between the two LP materials we offer.
The main differences are porosity and density though.
The synthetic felt materials are generally a softer material that your glass compresses into when polishing. This can give you a little more friction to allow for the cerium to polish more effectively for that material type. It will also generally tend to round sharp corners when polishing for any length of time. The softer more open structure of the synthetic felts is also intended to move the water away from your piece while trapping more of the cerium so you don't use as much to accomplish your polish.
The LP materials are an injection molded product specifically designed to be hydrophobic and highly aggressive. The LP66 material has a much higher hardness factor to allow for very sharp edge polishing. The LP material is engineered to move water away from your piece while maintaining a high cerium content on the pad. The pores in the pad allow for a higher surface tension and friction to accomodate faster and brighter polishing with less water and heat buildup.
We've been big fans of the LP material for a long time, but because of their structure, they will take longer to "charge" with cerium than the synthetic felts will. It is this structure that also allows them to perform much longer with more abuse than the synthetic felts also.
We believe, long term, the LP materials are a better value, but a lot of it boils down to personal preference in material use.