Some time ago I applied for internship to two companies (let name them X and Y) which create project in field I especially interested in. In both of them I passed all recruitment stages so I was just waiting for offer (I hoped).
Company X responded me and give me an opportunity to work with them.
Company Y told that they will call within an week (but I am not sure if their decision will be positive for me).
Currently I am convinced that I want to take opportunity in company X, but I don't want to burn bridges with Y because within year I want to try to get a job in company Y.
How can I (eventually) diplomatically reject an offer from company Y and express willingness to cooperate in the future?
Why I want to (eventually) reject offer from Y: I am currently 1'st year computer science student. Company X offered me a internship for summer for full time and next 3 month (during my second year) job with 1/2 time. Company Y want to full time summer and [3/4 to 1]* full time on my second year. I know how challenging my studies are, so I don't want to get such big part in second year. In third year I will be able to do this (because there is less work from faculty) so I want to delay company Y to the third year.