Is MacLea or McLea the correct spelling?
To begin with, what is the difference between "Mac" and "Mc" at all?
Fundamentally, Mac and Mc are interchangeable. They represent the Gaelic patronymic prefix meaning "son of."
Mc (or Mc/Mc with the c raised and/or underlined) can be considered an abbreviation or shorthand version of Mac, but in Gaeldom they are both often pronounced as "mack." My oldest family records all use Mc, but there were time periods when Mac was found exclusively in the old parish records as well.
So why do people care if it is Mac or Mc?
Well, over time it appears that Mac has come to be more common in families of Scottish extraction and Mc in families of Irish extraction. Given the pan-Gaelic culture once found across these two places and fluidity of English spelling through the ages, it is unlikely that this ever had any real significance. Modern surname books in Scotland tend to use Mac, as a conscious identifier of Scottishness, and the reverse in Ireland, but the reality is that they are the same and only in our modern era of spelling rules and written language is there any real difference, and today it seems to mostly be because of computers, etc.
Still, today, it is often seen that families will insist loudly on one spelling or the other for this very reason. My MacCartney family (Scottish) and my own MacLea family are both good examples of this. Still, many people in both families did not follow this convention.
So which is it? McLea or MacLea?
Me, I don't find this question very relevant these days. It's whichever way your family spells it.
The evidence of my research is that that the family name is traditionally spelled as either MacLea or McLea. If anything, McLea is more common in the old records than MacLea, but since as noted above they were considered to be interchangeable abbreviations or shorthand, this is not surprising.
There is some evidence that my great-grandfather Peter Richard MacLea believed the Mac spelling to be the correct one. All of his older children (my grandfather and his two sisters) went along with that, but most of his siblings and his younger children all used Mc.
Similarly, the children of Peter's brother Alexander seem to have had some idea that Mac should be construed as the "correct" spelling, though they generally spelled the name McLea, and this spelling eventually stuck.
Due to the idiosyncracies of descent, my family is now the only branch of our family that spells the name Mac because all the other families either lost the name due to marriage or used the Mc.
There are also tales that my ancestor Peter Richard, though born with the name MacLea, may have fudged the spelling of the name as "McLean" (with two lines under the c) for an application for his railroad job in Boston, which was then dominated by the Irish. Afterwards, when he got the job, he dropped the "n" but kept the Mc and always understood that the underlined 'c' was an abbreviation for Mac.
As an aside, it should be noted that there is no connection between the name McLean and McLea normally. In Glasgow, Scotland, and later in America, the name McLea has in very rare circumstances come to be mispelled as McLean. However, in the regions of the old country where McLeas are common, one would never have been mistaken for the other.
One more common mispelling for the name, even in those parts of Scotland, was McLae. This is only seen in records from the early 1800s.
Origins of the name
So, now that I've explained the reasons why the name might be spelled either way, it begs the question: Whichever way you spell it, what does Mc and MacLea indicate? Son of Lea? If so, who was Lea?
It's not actually that simple, and the Gaelic linguistics involved are somewhat complicated. As such, the origins of the name are discussed in a separate article.
Simply put, it is believed that this patronymic name went from an early form of a conventional patronymic name (perhaps meaning "Son of Dunsleve" or an occupational form of the patronymic for hereditary families of medical professionals, meaning "Son of the Physician"), through an intermediate stage of linguistic development, in which the spellings McOnLea and similar were common, until developing into its current form.
So, the name would have gone from something similar to Mac Dhuinnshleibhe (son of Dunsleve), perhaps with Mac an Leagha (son of the physician) as a nickname/professional name, through an intermediate form of Mac On Lea, to its current form Mac Lea. More detail will be included in the separate linguistic article as it becomes available.
Livingstone
And to make it all more confusing, the Chiefs of the Clan MacLea, and others, from the 18th Century onward, largely abandoned the McLea name and took on the name Livingstone or Livingston. Whether this is because of a familial relationship with the lowland Scots family of that name, or some affection, or through an effort to "Anglicize" the name following the 1745 Jacobite Rebellion (which found McLeas on the losing side), is unknown. However, more McLeas today have the name Livingstone.