First of all the tartan that I want to go with is the modern Gunn tartan. Being a McWilliams, I believe this to be the best choice since the Gunn tartan is more accessible than the Mac William tartan is.
Since I have quite a few Williamsons/MacWilliams in the family tree, let me offer a bit of personal experience: not all Williamsons in Scotland are affiliated with the Clan Gunn; the Mackays, Macleods and Macfarlanes also lists them as septs, although I would daresay that the majority of Williamsons in Scotland have no formal tie to a Highland clan. My Williamsons came from Fife, which was not part of "Gunn Country".

It may not matter to you, and that's more than fine with me -- wear what you want -- but I just wanted to point that out that the Gunns do not have a monpoly on the surname Williamson/MacWilliam.

In fact, you might want to consider the Flower of Scotland tartan, which was designed in honour of the late Roy Williamson, the Scottish folk singer.

Regards,

Todd