Well its really your perspective as to which one is your favorite... and at the risk of seeming long winded as usual..I would like to give you a little review and you can make up your mind here in this thread versus going to each gallery and trying to find the exact image that can sway you..
I like the Robovision version from Target, its got a nice lighter blue coloring and metallic paint which makes the fanboy in me so very happy. Its smaller and more in line with the movie concept, being as how he is skinny and all. The dual guns also match the video game and is kinda cool too. Seeing as how we never saw those canons in the movie its not entirely impossible for him to not have them, considering that non of his toy incarnations came with his energon laced blade...

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Then there is the regular released version of Voyager Prime in the original colors, whcih to me is nice as well. It's a little more darker in coloring and as far as I know there is no difference in the mold between this and the Robovision version..That said, the robovision suffers from some detailed paint that was missed on the upper leg/thigh area, and the shoulders, but its easy to add that paint in or just ignore it as I do..
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Now not to be outdone, is the 39.99 Leader class Optimus Prime that you speak of. He is a nice figure indeed, and has a fair amount of articulation. though not as slim and sleek as the Robovision or the Voyager Prime, the leader class has his nice features as well. His transformation is more movie accurate and he doesn't suffer from a giant back pack and his thigh area is kinda shaped wierd which from certain angles makes him look off, not to mention the thigh kibble that impeads his positioning as well... His chest lights up and makes noises which is nice and the automorph features are cool too. his size and articulation are what makes him a great buy. His blocky shape is his only downfall, that is if your looking for movie accuracy in terms of proportions..Overall though, leader class is a nice figure.
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though I own both the Robovision and Leader class I still hold out hope for them to make a Masterpiece for the high end Adult collector, but I will pick up the normal colored Voyager when I see him, and soon the Night watch as well, I'm a sucker for prime...Secretly though, I love the Robovision version..
As for which one to buy, well they have their good points and Bad points.. I just hope this helped you make a decision..