All other things being equal, Coke in glass and Coke in plastic will taste the same; unfortunately, things are often not equal and glass preserves the flavor much better than plastic, which is permeable and, if not well handled, can allow the contents to develop off flavors much more quickly, as well as sometimes imparting its own flavor as the plastic degrades.
Even glass, however, isn't necessarily protection enough for badly handled pop. Like many, I prefer sugar-sweetened Coke to the corn-sweetened variety. The difference isn't so much flavor as mouth-feel, though in a sense sugar-sweetened Coke tastes "brighter" than the more viscous corn product.
However, I often find that Mexican Coke has been maltreated in its journey north. Glass won't protect it from being exposed to overheated conditions, for example, or too long a time in the bottle and then it develops a funky taste. Also, based on the bottle labeling and what my palate sometimes tells me, some Mexican Coke has corn sweetener, too.
I usually stock up on kosher-for-Passover Coke in the spring. Then you can be sure of getting reasonably fresh sugared Coke, although, unfortunately 2-liter plastic bottles are typically the most readily available format.