XP is based on quality achieved through the actual synth process itself, so for example:

For Synth A you use 2 HQ mats and start with a Quality of 300, and end with a Quality of 400 (difference of 100)

For Synth B you use 0 HQ mats and start with a Quality of 0 and end with a Quality of 200 (difference of 200)

Synth B will give you more XP than Synth A.