Cognizant (CTSH) has historically gone 10%-20% faster than Infosys (INFY), and continues to do so - if we go by the guidance of the two companies for 2009. Until last year, its stock was trading at substantial premium to INFY. Since then, the differential collapsed. In the last two weeks, INFY has been trading at 10% premium to Cognizant. My guess is that the Indian markets are on a tear and are pulling Infosys India stock along, so the INFY ADR is duly obliging. However, Cognizant is not listed in India. So CTSH doesn't have something similar pulling it forward.
Whatever factors impact INFY will impact Cognizant to the same degree. If tech spending recovers, INFY will benefit, and so will Cognizant. Rupee appreciation hits both. US protectionism hits both. Amongst the Indian outsourcing companies, CTSH probably does the best quality of work - if one were to go by the number of IIM grads that chose CTSH over INFY, Wipro (WIT) or TCS.
Cognizant has grown faster than INFY in the past, and it will grow at least 10% faster than INFY in 2009. I think CTSH will again start trading at a 10%-20% premium to INFY after another two quarterly reports. INFY will report next in mid-July and Cognizant in August. So CTSH will problem reassert its premium by October or November.
Disclosure: Author is long CTSH and short INFY