The piece that both stars wore is a silk, patterned 'Louis Vuitton' button down. Adorned with cranes and cherry blossoms, the shirt is quite festive and perfect for the summer time. Song Joong Ki wore it on the cover of 'Harper's Bazaar' while Rain wore it for a 'Cosmopolitan' photo shoot.
Song Joong Ki styled it a bit more casually, pairing the shirt with baggy jeans. Rain, on the other hand, appears to be wearing the whole outfit as a set, which is composed of the shirt and thigh-length shorts. It's difficult to comment on Song Joong Ki's choice of footwear as his feet are cut off; however, as for Rain, we can see that he has on a pair of bright white slip-ons.
I know I'll most likely be going against popular opinion, but I have to say that I much prefer the styling on Rain. His entire outfit seems more fitted and suave whereas on Song Joong Ki, the shirt is a bit too large. Plus matching a shirt like that with a pair of baggy jeans just looks like sloppy work. Song Joong Ki looks like he's ready to hit the hay...but maybe that was what the stylist was going for.
In any case, I really like Rain's clean-cut look; sorry, Song Joong Ki!