I wouldn't recommend either, but to answer your question, Megahorn would be the better choice.
The reason is that Poison Jab generally has no usage and only hits 2 types super-effectively: Fairy and Grass. Rapidash can already hit Grass-types hard with its STABs, while Fairy-types are literally non-existant in the tiers that Rapidash is used in (NU and PU). On the other hand, Megahorn can hit a wider variety of types, such as Dark-types and Psychic-types. This can be used to deal with certain threats, like Mesprit, Xatu, and Malamar.
Although many might say that the Megahorn has lower accuracy, it is important to note that Poison Jab has 80 BP with 100% accuracy, while Megahorn has 120 BP with 85% accuracy. That is a decrease in accuracy of 15%, but the BP increased by 50%! It's safe to say that the exchange in power and accuracy is another reason why to choose Megahorn.
To improve on your moveset, I would change it to something like: Flare Blitz / Drill Run / Megahorn or Wild Charge / Wild Charge or Will-O-Wisp (don't have the same move twice). Flare Blitz is used for extra power. Drill Run nails Fire-types that try to wall it. Megahorn's explanation is stated above. Wild Charge is extremely important because it deals with Water-types (which is what Rapidash often struggles with). Will-O-Wisp cripples physical attackers.
Hope I helped!
