What's the difference between fresh and bottled lime juice in cocktails?

Massive — fresh-squeezed lime juice is bright, acidic, slightly sweet, with citrus oils. Bottled lime juice (Rose's, ReaLime, etc.) is pasteurized, often sweetened, and tastes flat, candied, and one-dimensional. Use fresh for any cocktail.

Bottled lime and lemon juices are processed for shelf stability, which destroys the aromatic compounds that make fresh citrus taste like citrus. Specifically: (1) Pasteurization — heat kills enzymes and microbes but also breaks down the citrus aldehydes and terpenes that give fresh juice its bright nose. (2) Sweetening — many bottled limes (especially Rose's) include sugar and preservatives, which fundamentally changes the recipe ratios. A drink calling for 1 oz fresh lime juice and 0.75 oz simple syrup goes wrong if you substitute Rose's; you've doubled the sweetness without adding any extra acid. (3) Fragility — the citrus oils that come from the peel into fresh-squeezed juice (because you squeezed it minutes ago) are entirely absent from bottled juice. Practical advice: fresh-squeeze citrus at the start of the drink-making session. A handheld citrus squeezer (the metal hinged kind) is $15, lasts forever, and yields about 1 oz of juice per lime. Squeeze what you need and use within 4 hours; freshly squeezed citrus juice oxidizes and loses brightness over 24 hours.