When it comes to espresso based hot beverages...I think that Soy Lattes are the best. I do not drink soy milk on its own and I do not have an anti-dairy bias so I consider myself to be an objective latte evaluator. My reasons for asserting the superiority of the soy latte:
Taste: Most importantly, I think that they taste better than milk lattes. Soy milk has a natural sweetness that complements the shot of espresso. I was surprised to learn (via the Starbucks website) that lattes made with soy and 2% milk have identical quantities of sugar in them. When purchasing a regular latte I find it necessary to add additional sugar to the drink. Additionally, there is a slight nutty taste that adds to the drink.
Consistency: Soy milk is a bit thicker than 2% and I think that this adds a richness to the drink. If you taste test them I think you will find that the milk latte is thin and runny by comparison.
Nutritional Info*: (As compared to a 2% latte which is the default at Starbucks and many chains) The soy latte has slightly fewer calories. There is a substantial fat difference, however. The soy latte has 4.5 grams of fat with .5 grams of saturated fat. The 2% has 7 grams of fat with 4.5 grams of saturated fat. The 2% wins on protein with 12 grams vs 9 grams for the soy. These figures assume that additional sugar is not added and as stated above, I find it necessary to add sugar to the 2% in order to achieve a commensurate level of sweetness.
A note about Foam: I also find soy foam to be superior to milk foam. This is mainly because of the sweetness discussed above. Even when I add sugar to a milk latte, it does not get incorporated into the foam. It is also thicker and more substantial than milk foam. Ironically, the soy foam tastes like a closer cousin to whipped cream than milk foam.
Store Brand: I prefer the Starbucks Soy Lattes to those at Peet's (the espresso taste is overpowering) and Coffee Bean (can't quite remember why as I have not purchased one from them in awhile.
Conclusion: While I have compared the Soy Latte to its 2% based relative, I also prefer it to mochas and other flavored espresso milk based drinks. My main concern has been taste, however the added sugar in these drinks heavily tips the nutritional debate in Soy's favor. Many people have an anti-soy bias (when at Starbucks with a friend, he referred to my drink as very "Berkeley"**) and I would encourage those who have not tried one to give it a shot.