The two options are so different its hard to compare them.
To me Lancelin is about the full on windsurfing vibe (like Kan-aha in Hawaii) the warm water, lots of people to sail with, wearing boardies while sailing, euros doing crazy freestyle tricks on the inside and blasting through main break on slalom boards (its been a few years since I was there).
Tassi is more about the variety of locations, the changeable weather, the quietness of the beaches, exploring and sailing new spots, there are just so many places to sail most of them pretty much deserted except for the occasional fisherman with a three meter shark in a four meter tinny, a few dog walkers and lots of oldies in their two hundred thousand dollar campervans who want to tell you how they surfed Bells back in the fifties on a surfboard they had carved out of an old railway sleeper.
There are definetly more people on the water during Jan and Feb, Marrawah can almost get crowded with up to four people on the water at the same time. If you go to Stanley you might even see that rare species, the Kitesurfer!!!! ( Parks and wildlife have issued a protection order in the hope that they breed)