Thanks for the question, Rob.

The easy answer is that yellow tomatoes are a little lower in acid and a little bit sweeter.  The truth is though, that there are a lot of varieties of tomatoes and sweetness and acidity can vary a great deal.  

That brings up your "Heirloom" question.  Heirloom breeds are generally understood to be varieties of open pollinated plants that have been raised for at least 50 years.  Over the  last couple decades, large farming operations have created hybrid breeds that are better suited to getting a huge quantity of consistent product to market.  (Consistent - not great!)

In general, "heirloom" tomatoes will be slightly imperfect in appearance, with a huge variety of color and shapes.  Your best bet is to buy local, as the term has started to be abused by agri-business firms.  Our local farmers' market is an excellent opportunity to find great tomatoes, though you should have pretty good luck anywhere in the U.S. between July and September.    

Bon Appetit!