To do another comparison, check what the Delta flight will cost you in FF miles vs. the actual dollar cost. FF miles are valued at from 1.25 to 1.75 cents per mile. Use the midpoint of that, so if flight requires 25k miles, then if it costs more than $375 ($0.015 * 25,000), you will spend less if you use miles than if you buy the ticket with your dollars. If flight is < $375, pay cash, and use miles for some other flight.
Some other factors for this too, how often you use FF miles -- not often, then little emphasis on the above comparions. Travel internationally? you typically get more out of your FF miles than domestic flights, etc. Rarely use the miles? then skip the above comparison, because holding miles in your account for long perions of time, say five to ten years, means they *will* be devalued over that long a time period (example is you earned X miles, and they are sitting in your account. Those miles can get you two international flights in business class today, but wait five years, and airline raises the number of miles needed to get those two business class international flights, and so your miles were just devalued. Worse, the miles expiration after Y number of years, ... serious devalutaion).
A basic rule is use the miles as you earn them, don't let them sit around, but don't use them for things you can get for less for if paying with dollars.