Is it better to do long-term preparation for JEE rather than joining in an EAMCET college?

If you are interested to get a seat in NIT then choose long-term preparation, otherwise, you can choose a college under EAMCET. Ultimately, it depends on your interest.

Taking a break for a year sounds good as it means 12 months of extra time for preparation and a second chance at IIT or NIT admissions. Since one whole year of dedicated study exclusively devoted to JEE Main sounds very promising of a very good JEE Main score, you may think of this as a good option.