Id say megatrin for the simple fact theres no first edition of him. If you wont buy the first editions of the rest because of price theres always the chance hasbro will release them eventually at retail again. At the very leadt doing it that way gives you a chance to think on which of the other voyagers you want as either FE or prid. I bought megs and dont regret it. Apart from his backpack he is a really cool toy.
Both are very cool, but if you can only get one...I'd say Megatron. His transformation can be a real bother, but he's super cool in robot mode. Optimus Prime is also cool in robot mode, and his vehicle mode is what you'd expect from him. The FE Prime looks soooooooo much better though.
Well if we wanna be technical, there's already 3 versions of the FE mold. It's just that along with being deluxe sized (be it on the larger end of deluxes), all of them are rather expensive, while the PRiD Voyager is readily available at US retail, and that has its own positive qualities to boot.