Over the past month, Celgene (NASDAQ: CELG) shares notched a gain of more than 20%, while Merck & Co. (NYSE: MRK) stock has risen about 17% year to date. Both drugmakers have exciting futures in the oncology space. With worldwide cancer medicine spending projected to rise from $107 billion last year to over $150 billion by 2020, investors are right to wonder which stock is best positioned to ride this trend into the clouds.