First, I will briefly explain how mortgage payments work. Then we will look at how the two different options you have affect those payments. 
When you take out a mortgage, you have borrowed a sum of money at a certain interest rate. The interest on that amount of money needs to be paid by you every month. If all you did was pay the interest, you would never pay off the loan. Instead, in addition to paying the interest due, you are also paying some amount toward the principal of the loan. The next month, your interest charge is a little less than the previous month (because there is less loan principal accruing interest), and a little more of your payment goes toward the principal. When you first took out the loan, the bank calculated exactly what the monthly payment needed to be so that the monthly interest charge would be paid and your loan would be paid off in 25 years. 
When you make an extra payment toward your loan principal, the interest charge next month will be less, because the size of your loan is less. As a result, the bank is offering two different options:
- Option 1 is to keep your monthly payment the same. But because your interest charge is less, more of this payment goes toward reducing your principal. As a result, your mortgage will be paid off quicker.  
- With Option 2, the bank uses the savings in interest charges to lower your monthly payment. The monthly payment is recalculated so that your loan is paid off in the original 25 years.  
Now, to your question: Which option is better?
Option 1 will save you more interest over the life of the loan and will get you out of debt quicker. 
The advantage to option 2 is the lower monthly payment. However, because the length of the loan is longer than with option 1, it will cost you more in interest. 
It seems to me that you are not having any trouble making your current monthly mortgage payment. (You were able to save up this extra lump sum to apply to your mortgage.). Because of this, I recommend option 1, which saves you money and gets you debt free sooner.