Why is everyone advising to choose the NIT over the BITS Pilani (Goa campus also included)? Is the BITS Pilani computer science at least as good as a new generation IIT?

Don't pay heed to the ones advising you to choose NITs over BITS Pilani because no matter how good even the best NIT Trichy is, it can never reach the level of globally recognized BITS Pilani. BITS Pilani is second only to the 5 old IITs and every other technical institute in India comes after it. 

The usual preference order is

IIT Bombay>IIT Delh>IIT Madras>IIT Kanpur>IIT Kharagpur>BITS>IIT Roorkee>IIT Varanasi>IIT Guwahati>NIT Trichy>NIT Surathkal>NIT Warangal

It's very important to evaluate a college's worth and credibility based on relevant factors. Here are a few aspects you should take into consideration while making the decision. 

• Faculty and Pedagogy
• Infrastructure
• Labs
• Library
• Academic rigor and facility
• Placements
• Student body
• Fests and clubs
• Hostel and campus facilities
• Mess food

Ultimately, it's up to you to prioritize one factor over the other and make a well-formed decision based on the research you have done.