Good decision I "made" / "have made" today

As a non-native English speaker, I'm a little bit confused. Should I say:

The only good decision I made today was to ...

or

The only good decision I have made today was to ...

What is the difference between the two sentences?

"I have made" is the present tense: you are describing the present, and in that present there exists a situation where there are some decisions that you have made, which are now in the past.

"I made ..." is the past tense: this is more simple, you're just describing what happened in the past.

There may not be any difference between these two in terms of meaning. Taken in isolation, there is no real difference, and both are equally acceptable.

In other cases the meaning may vary significantly. Compare "I made it to Paris." vs "I have made it to Paris." In the latter it sounds like the person has just got to Paris, in the former they could have got there at any time in the past.

The most important word here is probably 'today'. That gives what you say a precision in time that is more fundamental than the difference in the tenses. So long as 'today' is there the two statements are virtually identical and both will work.

English is often imprecise about time, with overlapping tenses, so a precise term like 'today' will nail things down better than any tense will.

On the other hand, there is a difference between "The only good decision I made today was to…" and "The only good decision I made today is to…" If you say "was to" it indicates that you already acted on that decision and there is no going back. If you say "is to" it indicates that you made a decision, but have not yet carried it out irreversibly.

So in that sense the statement "The only good decision I have made today was to…" sounds slightly awkward (because "have made" sounds less final as a tense, but "was to" sounds final). So I'd go with "The only good decision I made today was to…"