Are limes and lime juice more acidic than lemons and lemon juice?
I'm culling a lot of information from one of my favorite cocktail books for this one.

Yes, limes have a slightly higher acid content (on average) than lemons do - about 6% for limes, compared to 4.5% for lemons.  More importantly for their flavor, lemons have about 2% total sugar, while limes have somewhere between 0.5% and 0.75%.  Sugar/sweetness has quite a suppressive effect on the perception of sourness, so lemon juice will most likely taste a bit less sour than lime.

The composition of acids in the two also differ.  The acid in lemon juice is almost entirely citric acid, which also makes up most of the acid in limes.  However, limes include about 10% each of succinic acid and malic acid which have an effect on their flavor.  Malic acid is most recognizable as the sour flavor from tart apples and rhubarb.  About succinic acid, Wikipedia states:


  It lends to fermented beverages such as wine and beer a common taste that is a combination of saltiness, bitterness and acidity.


Personally, I think the difference is perceptible in beverages, but less so in cooking.

Culturally, lime would most certainly be the traditional choice for Latin American cuisine.  Lime is much more readily available in Central and South America (three of the biggest producers in the world are located there).  How the difference in acid content would contribute to the browning of avocados is more questionable.

EDIT: Here's an interesting paper, suggesting that ascorbic acid (vitamin C) is more effective at preventing avocado browning than citric acid.  Which is an interesting wrinkle, because lemons contain significantly more vitamin C than limes do...

EDIT 2: And along comes Jolenealaska, with an experiment that seems to undermine the entire assumption that acid reduces browning of avocado at all!  Seems like the real secret may just be keeping your avocados either uncut, or refrigerated and tightly wrapped if you have to cut them ahead of service.