Sir, I just completed class 12. Should I prefer long term coaching or join in any other private university. Which one is better

See if you have patience and also confidence to go for jee advance by just droping a year, you can. But if you're not focused in between the gap then again it will be hard time for you because you have just dropped your year. And also if you want to join any private university then you can go with that option, as your marks is well to get into good college.If you want to go for college, i can provide you link where you can predict your college by mentioning your marks, percentile and also preferred branch you want to pursue and category based wise:https://engineering.careers360.com/jee-main-college-predictor  

Is it better to prefer Sastra deemed university to join in computer science engineering.

SASTRA university is very good to persue CSE but their admission procedure is little bit different from other colleges. They also give more importance to 12th marks also.For admission in sastra university application form already started to fill and last date to apply is 14th june 2019.You can follow up the lini for more information about college and also for new updates for college:https://www.google.com/amp/s/engineering.careers360.com/articles/sastra-btech-admission/amp 

Thank you