HDFC Diners Privilege as well as Regalia Credit cards have many similarities. They have the same reward rate as well same bonus multipliers on Smartbuy.
How to use/maximize with these cards is totally different.
HDFC Regalia credit card is much simpler to use. For meeting the Annual Milestones you are awarded with reward points which I value more compared to the complimentary Memberships.
HDFC Diners Privilege credit card has major acceptance issues. You get vouchers as well as complimentary memberships on meeting monthly and annual milestones. HDFC Diners Privilege is a tad more cumbersome to use. But gives 0.5-1% higher savings. Add-on card holders also get complimentary lounge access which gives it an edge over Regalia.
Best way is to compare yourself by using the calculators given below.
In case the calculators have not helped make up your mind, the basic table given below might do the trick.
Conclusion
For someone who is just a casual credit card user, HDFC Regalia is better.
For someone who is cardgyani and will meet the monthly milestones, HDFC Diners Privilege is more lucrative.