Rather reduce the cost
There are not many uses for money in 5e. In every game I played, the party had more money then uses for very soon. This means that reducing the cost will get them holy water a bit sooner, but the point where it is available in large quantities already comes sooner rather than later.
Improving the effect on the other hand is somewhat risky. You would quickly get to where it is better than most other things people can do. It would also “steal” abilities that you usually get from your class or from magic items. Discussing this in detail is beyond the scope of this answer, but it could easily get out of hand. Now, currently the party can not afford large quantities, that’s your question after all. But later, more money will be available and they will be able to afford a lot of holy water even at 25 go.
In summary, reducing the cost seems like a minor risk while improving the effect would take a detailed analysis. If you want your players to have something better, you should give them consumable magic items that are available in limited quantities. If you want to go with improved holy water you should design the rules and ask a separate question about your idea.
As was pointed out in the comments, the ceremony spell consumes 25 gp of silver but produces a single flask as one option. This might not be relevant, but if any of your party members have access to the spell (they could, it’s 1st level) you should consider your changed price there, e.g. by having it produce an appropriate, larger amount of holy water.