Definitely joining IIM Calcutta over IIM Lucknow is better. In fact the huge difference in fees is exactly the reason why IIM Calcutta is worth investing in more than IIM Lucknow.
Some advantages of joining IIM Calcutta over IIM Lucknow are:
IIM Calcutta is among the top 3 while IIM Lucknow is below.
The placement opportunities at IIM Calcutta are way better than IIM Lucknow. Check the recent placement reports to verify the same.
Professors with publications in International journals are way higher in IIM Calcutta than Lucknow.
IIM Calcutta has a better batch quality as it has a higher cut off on CAT %ile.
IIM C Alumni networking and culture is enviable even by IIM A and B. So by paying more fees you build relationships that are priceless and last a lifetime.
I wouldn’t say choosing IIM Calcutta over IIM Lucknow will ever be a bad decision. Your 7 extra lakhs Rupees will pay you back tenfold times.