The high tensile parts on the Shogun won't last long and those cranks are suspect. I would go for this bike. The components are better suited to trials and it'll come with trials pads on the back brake. Super important for trials. The extra money is well spent.
i think for being 15 you would wanna stick with a 20. im 14 and i hate 26 inch bikes just becuase thay feel big and clumsy.
if you get the chance. ride a couple different bikes. ride a 20 then ride a 26. i think if your comfortable on a 26, they would be easyer to learn on. but i think you would have more fun on a 20.