Be careful chasing momentum, writes UBS' Javier Kulesz, reminding of 2 years ago when Brazil - garnering the Olympics and the The Economist cover while getting kudos for its swift post-GFC recovery - was a market darling, while Mexico was just the opposite. Since then, Mexico (EWW) is up over 20%, outperforming Brazil (EWZ) by more than 4500 basis points. Now it's Mexico drawing the raves. One might do worse going forward than going long Brazil against a short in Mexico.