Both are good choices. Through JEE Mains it is little tough because of the cut off. I suggest you to appear in JEE Mains and also if you want admission in UPES then appear for the UPES entrance test, but I will also suggest that if you want computer science then there are so many other good options for this stream. Like you can take admission in BIT MESRA and its off-campuses, Chandigarh University, etc..