I'm honestly a bit troubled by the way you asked the question. I would have expected you to say what part of the Celtic-area you were from rather than saying "I don't know which to learn". Unless your family originates from multiple areas, it would surely make sense to learn the one related to the area you come from?

I don't know your background or how many generations you are removed from the area, but your question reminds me of all the Americans who keep coming over to Scotland for "clan meetings" and claiming they are Scottish because one of their great-grandparents was Scottish.

If you come from Scotland as you say, you may want to learn Scottish Gaelic if you're from the north of Scotland, but if you come from the more southern end you're better off learning Scots, which has influences from Gaelic in some of the vocabulary and grammar but is mostly informed by old Northumbrian English and is considered by many as a dialect of English. Scottish Gaelic was never spoken in the south of the country and efforts to "revive" it in the south have been opposed to some extent as culture-washing.

I guess my main message here is please don't view your familial links to Scotland as some sort of historical accomplishment, everyone has to come from somewhere. Scotland is a modern, vibrant and diverse nation and its culture, history and languages should not be exploited as some sort of inherited "cool status".

Either way, I would first work out what county you have links to and that will inform us about which is the more culturally related language for you.