If ever there were two settings GURPS was meant to do, it's historical WWII... and Dark Sun. GURPS was built from a game called Man-to-Man that focused on gladiator-style combat, and it has superior rules for both magic and psionics, and it handles special weapon materials really well. It's also really easy to build a racial template in GURPS, making it a cinch to convert half-giants and such. Built at the right point totals, and using the right campaign lenses, GURPS is also appropriately deadly, yet without making PCs disposable, which sets, I think, just about the perfect tone for Dark Sun.
GURPS Lite is available to sample for free if you want to get a taste of its mechanics.