Phentermine 15mg vs 30mg?

I have been using Phentermine for weight management on and off for about 2 years now (prescribed by a doctor, approx 3 months on/9 months off over the course of each year).

I started on 15mg, then went up to 30mg after the first year, but now I have to go back to 15mg due to it being too expensive.

For those of you who have tried phentermine, would you say 15mg is still worth the money? I took it so long ago, I can’t remember how it compares to the 30mg. The higher dosage is out of my price range right now, so do I get the cheaper 15mg, or do I save my money for bit longer to afford the 30mg?

Any comments appreciated!